Hearthpwn

Flame Imp
Great 1 drop. Can trade upwards, but is very fragile. Best played early, obviously. One is more than enough tho, as the self damage can get out of hand if you also aggressively lifetap. 

Mulligan: There aren't many instances where if you don't want him on your start hand. Always keep.
Replacements: Argent Squire, Clockwork Gnome

Voidwalker
Very solid and versatile 1 drop. Can be used throughout the whole game as a means to protect your important stuff and absorb a hit from a weapon or minion. 

Mulligan:  DON'T keep against Priest unless you start and have Abusive Sergeant on the hand. DON'T keep against Warrior unless you have stuff to protect like a Knife Juggler or Flame Imp. 
Replacements: -

Haunted Creeper
One of the toughest minions around. Provides a plethora three enough targets to make use of your buff cards. I decided to only play one, as two slowed down the game too much against control decks. It's not that great late into the game and on its own, so only one is reasonable enough to play.

Mulligan: Keep against other Warlock (Zoo), Mage (Tempo / Mech), Hunter or if you have no other 2 drop in hand.
Replacements: Argent Squire, Annoy-o-Tron, Harvest Golem

Nerubian Egg
One of my favorite cards design-wise. Weak on it's own, but with an incredible reward if used properly. It's a perfect target for your combo cards and nets you much street cred in form of a B.A. 4/4 Nerubian. Also a M.A.D. deterrent against board clears, so you still have something on the board, in case your opponent thinks about it. Due to its effect, you opponent probably won't kill it willingly in most cases, so it can be droped pretty much anytime in the game (if you have no better play) in preparation of using it for a combo card several turns later. 

Mulligan: Keep if you have Abusive Sergeant or Power Overwhelming on hand. Keep if you have no other 2 drop, otherwise don't keep against Druids, Priests, Hunter and Warriors.
Replacements: -

Imp Gang Boss
Kobe steak of the meat cards. Motto value. Much advantage. Have BRM? Use him.

Mulligan:  Keep when he fits the curve, so pretty much always keep him, unless you play against a heavy control deck and have a Piloted Shredder or Voidcaller (w/ big demon) AND coin in hand.
Replacements: Harvest Golem

Dr. Boom
Very good late game card you pretty much always drop in most situations. Makes the deck more consistent in form of a big threat that also generates tempo. 

Mulligan:  Always mulligan away. You don't need him before turn 7.
Replacements: Sylvanas Windrunner, Ragnaros the Firelord

Piloted Shredder
I played variants with and without this card, in combination with and without Voidcaller w/ demon engine and it brings much consistency. It adds a ton of pressure. 4 attack is nothing to sneer at and it serves as another meat card due to the stickiness. Also draws silences like teen panties to a boyband. I said it before GVG and I say it again, one of the strongest "new" cards in the game.

Mulligan:  Keep if you have coin and face a control deck / class like Priest and Warrior.
Replacements: Hard to replace, Dark Iron Dwarf, Shattered Sun Cleric

Voidcaller
Great comrade. IF you run Voidcaller, you also need to run a demon engine, which I can highly recommend.  As I said before, I tested this particular deck without a demon engine and while it works, it works not as good. It has solid stats and provides a lot of tempo and can single-handedly carry you to victory. Can net you ridiculous advantage if traded up and summoning a Doomguard or Mal'ganis. Even an Imp Gang Boss is great value. Also a great deterrent against board clear.

Mulligan: Keep against control decks like Warrior and Priest if you have Coin and a big demon in hand. Also decent against Paladin.
Replacements: -

Defender of Argus
Makes big guys bigger and small guys not as small. He is the protein to your deck and is incredibly versatile and a much needed defensive card. Giving tokens taunt and preventing them from being pinged or killed by other tokens, is a great way to ensure you have enough breathing room to bring your opponent to low health and forcing him to trade into you. That is the reason you don't always want to buff up big guys, it makes them more susceptible to minion / buff /weapon removal.

Mulligan:  Never keep unless you face hunter.
Replacements: - 

Power Overwhelming
If Defender of Argus is the protein, Power Overwhelming is the steroids of your deck. Primarily used to make weeny tokes into 'roid swallowing tools of mass destruction, which enables you uber-efficient trades with high mana minions, it's also a great finisher.

 Mulligan: Keep against Druid (optionally for Priest). Keep with Nerubian Egg. Otherwise mulligan away.
Replacements: -

Abusive Sergeant
Similar to power overwhelming, he is used to enable you to trade efficiently. Not much else to say about it, really.

Mulligan: Keep against Zoo and Hunter if it's your only 1 drop. Keep with Nerubian Egg, Voidwalker and Haunted Creeper.
Replacements: Direwolf Alpha, Dark Iron Dwarf

Knife Juggler
One of the most solid 2 drops in the game. Can net you great value, offers decent tempo and is a high priority target for your opponent. It's not utterly important, but he has little competition.

Mulligan:  Keep in most cases.
Replacements: -

Imp-losion
God I hate this card. I ran two for some time, but ultimately got screwed way too often. So as a personal flavor, I decided one is enough to cope with. However, if it doesn't screw you, it offers lots of tempo, gives you lots of tokens to make use of and has direct synergy with Knife Juggler and Mal'Ganis. Of course I tested the deck without it, but quickly came to the conclusion that one is somewhat needed.

Mulligan:  Keep with Coin against Zoo. Decent against Mage  (Tempo / Mech) and Paladin. 
Replacements:  1 is needed, Bane of Doom

Bane of Doom
Bane of Doom is pretty much a make it or break it card, but in favor of being good. 
Due to the nature of the deck and the fact that you should always have board control, even IF it spawns just a Flame Imp it should not break your board presence too much. However, if it works, it works VERY well. In fact, it will always generates tempo - just in different nuances. 

Mulligan: Never keep.
Replacements:  Imp-losion

Bomb Lobber
Probably the biggest surprise of the deck, but if we're talking TEMPO it doesn't get more tempo then dropping mad bombs like it's WWII. A 3/3 with an inherent Flame Cannon build in, how can you say no?! Seriously, I tested him quit a bit, out of a whim and I was surprised how effective he is and kept him ever since. Since you have board presence in most cases, you can "set up" the board so he nets you full value all the time. Also tried to run two and while it was successful, I felt it was a bit too situational to get full value out of both, since it's not a card you want to play into every board state. Also dope to kill a Shade of Naxxramas and is a clutch comeback card. Try it out.

Mulligan: Never keep.
Replacements: Fire Elemental lol, Dark Bomb, Bane of Doom, Imp-losion 

[card]Doomguard[/card]
Doomguard sits usually in hand waiting that it either doesn't discard valuable cards, is summoned via Voidcaller or used as a finisher or set up lethal through sheer pressure.
It's versatile enough to warrant to be played on its own, although I would only run 1 in that case, unless you play a very low-curve version of aggressive zoo.

Mulligan:  Never keep unless you have Voidcaller with Coin against control decks and Paladin.
Replacements:  1 is needed. Floating Watcher, Dread Infernal

Mal'Ganis
Mal'Ganis is actually pretty underwhelming on its own. Only voidcaller makes him worthwhile to include. He is a lot more straight forward than you would think. If he is on the board you can tap for free and apply huge pressure. He is also the biggest priority target you have in your deck. It's also important to know that whenever he hits the board, your opponent has a 80% chance to automatically transform a card into a Big Game Hunter. Just make use of him as long as he is on the board, which in most cases won't be long. Don't play around him, by trying to set up Imp-losion or something.

Mulligan: See Doomguard.
Replacements: Doomguard, Floating Watcher, Dread Infernal 

Ironbeak Owl
The STFU of the deck. I always felt one Ironbeak Owl is not enough, while two might be too much, but since it's either one or the other, I would rather run with two. You have enough targets to warrant them and having one at hand can be crucial in many situations. It's often a MVP. Every Class has at LEAST a few targets: Sylvanas, Mad Scientist, Nerubian Egg, Haunted Creeper, Sludge Belcher etc

Mulligan: Always keep against Warlock (either deck), Hunter and Mage. Decent to have against Priest, just in case.
Replacements: -

Loatheb
Tried to play with and without him and while you will not actively miss him if you don't, you do appreciate it IF you do in many situations. He protects your board against clears, sets up lethal and prevents an OTK from your opponent - all that paired with a very solid body, so that even if you don't get any use out of the effect, he doesn't end up being a dead card.

With Malygos-Lock, Combo Druid and Warrior running rampant in EU, it's a great tech card to include. Also great against (Freeze) Mage, Rogue and to prevent BM lethal from Hunter.

Mulligan: Never keep.
Replacements: Bomb Lobber, Sylvanas Windrunner 

Mulligan:

Normal mulligan. Keep Ironbeak Owl as it's a vital tempo-thief against Mad Scientist or an overblown Mana Wyrm and can be used against various targets in Mech-Mage, like Piloted Shredder. Having Nerubian Egg + activator is usually a great way of overpowering his early game removal.

Playstyle:

vs Mech:
Fight hard for board control and kill priority targets like Mech Warper ASAP. You should ususlly gain the advantage by turn 5 - 6 and can then ride the win home by keeping his board clear with efficent trades and applying pressure, if it makes sense. The match can be over quickly for both players. If he draws everything and has RNG Mother Russia on his side, it might be difficult to get the board back. It can happen just as likely in your favor, tho. Overall a pretty decent match up.

vs Tempo: 
Similar to Mech. Make sure to get rid of Flamewaker and you should be more or less fine.

vs Freeze:
Probably my least favorite match up and the true cancer of the game. It's a drag, has little to no interaction and requires little effort. Unmanliest, most coward deck in the entire game... of all TCGs... IN THE ENTIRE UNIVERSE. Do not approve. You want to apply pressure hard and go for face whenever possible. As long as you are above 15 HP, he will Alexstrasza you anyway. Only exception is Acolyte of Pain, you want to kill it so he can't ping it trice. Perfect silence target, along with Mad Scientist and less desirable Loot Hoarder. Only trade if it nets you more damage in return (e.g. saves a higher attack minion). Loatheb is a key figure in this match up. You want to save him until you are within Lethal range and can burst him down with Power Overwhelming and/or Doomguard. 

Mulligan:

You want to actively mulligan for Ironbeak Owl against Mad Scientist and less desirable Haunted Creeper. Voidwalker and Knife Juggler are other cards you are looking for. Defender of Argus doesn't hurt to keep, if you have a few early drops. Imp Gang Boss is also a strong contender.

Playstyle:

vs Face / Hybrid: 
Play Abusive Sergeant if you have no other 1 drop and no Nerubian Egg in hand. You want to mitigate as much damage as possible, while keeping board control. Play around Freeze and Flame Trap. Sometimes it's advisable to not attack if you only have a high level creature on the board. You can taunt it up and just pass the turn. Once the early turns are over, you can usually outpace him and apply more pressure than he does and burst him down.

Mulligan: 

Don't bother keeping Abusive Sergeant without target and likewise Voidwalker without Abusive. Haunted Creeper is pretty meh without the other 2 cards or Flame Imp, since that means free draws if he has Northshire Cleric. I usually like to go for higher attacking cards like Knife Juggler, Flame Imp, Piloted Shredder, Voidcaller, Imp Gang Boss and the like.

Playstyle:

vs All:
Assume it's control priest first and foremost and Mech / Aggro Priest second. You have enough tools to deal with the latter naturally. There is not much to the match up, be prepared for Auchenai Soulpriest + Circle of Healing at all times, Holy Nova and Lightbomb. Don't overcommit too hard without sticky minions. Be careful with Imp Gang Boss late into the game due to Cabal Shadow Priest and the possibility of Shadow Madness killing off your Voidcaller.

Mulligan:

Normal mulligan. I like to keep one of the holy trinity on hand, in particular with Coin. Voidwalker is utmost desirable as it deals formidable with dudes and absorbs a weapon hit.

Playstyle:

vs Control
Keep at least one Ironbeak Owl always in hand against Tirion Fordring. The other can be used freely against Piloted Shredder, Sylvanas Windrunner and the like. 

vs Mid / Aggro
Kill Knife Juggler and kill dudes if you can do so effectively due to Quartermaster. That means, don't bother running your Knife Juggler or Flame Imp into Muster for Battle tho. Just apply pressure in that case. You should have the upper hand in most cases, as you have a better transition into the mid and late game, while being able to keep up with the early game.

Mulligan: 

Normal mulligan. Don't bother keeping Owl. Voidwalker and Haunted Creeper and Imp Gang Boss are nice to have along with Power Overwhelming, to get rid of Violet Teacher asap. Flame Imp is usually only good to absorb a Backstep, to save your Knife Juggler. 

Playstyle: 

vs Oil
The matchup where Bomb Lobber yields the biggest value. Since Rogue doesn't play many minions, his lonley Azure Drake makes a perfect target. Loatheb can protect your board and prepare lethal and set him under pressure. You should be able to apply enough pressure that he has to use his weapon buffs and damage spells as removal. Keep Defender of Argus to prevent burst attacks towards the mid and late game.

vs Mill
Vomit your hand onto the field, kill Coldblood Oracle to prevent him bouncing it back. Play Loatheb. Profit.

Mulligan: 

Due to the slow nature of Warrior, go for Piloted Shredder and Voidcaller, in addition to Flame Imp, Knife Juggler. Voidwalker is okay-ish, but only to absorb Fiery War Axe.

Playstyle: 

vs Get in here 
The key is to apply pressure HARD. You want to overwhelm him, so he has to use his combo pieces to deal with your board. Try to avoid having too many <3 attack creatures towards turn 8 (6 with Emperor). If you fall behind, it's difficult to get back, but overall you should be able to force his tools consistently, which negates much of his combo potential. If he has a full pateron board, kill Warsong Commander first. It's your highest priority target. Use Ironbeak Owl on Acolyte of Pain.

vs Control
Get your stuff out and attack hard. Keep Brawl in mind, so don't overcommit and for istance, play Dr. Boom on turn 7 if you have a full board. Rather keep it in hand and play it after Brawl. Voidcaller setups can prevent you from losing tempo against Brawl turns, along with other stick minions. Use Defender of Argus and Voidwalker to make his weapon less effective in trading and to prevent that your buff dudes get damage for Execute. Power Overwhelming is crucial towards the mid and late game to effectively deal with Shield Maiden, Sludge Belcher and co.Ironbeak Owl usually finds Sylvanas and Belcher as target. One of my favorite match ups personally.

Mulligan: 

Normal mulligan minus Void Walker and Haunted Creeper.

Playstyle

vs ???
You have to decide when to ignore totems and when to go for face, but you should always kill the spellpower totem. You can usually overpower him and keep board control. Due to the lack of draw, he will run out of steam fast.

vs Mech
Same as above, with the exception of swooping down on HUIIIIIIIIIIIIIIIIIIIIII Whirling Zap-o-matic like it's a bowl of quark. It needs to die. Fast. Mech Shaman with perfect draws can kill you very quickly, so try to get stuff onto the board to deal with their stuff.

Mulligan:

Don't bother with the low attack stuff. Flame Imp and Knife Juggler are okay, otherwise go for Piloted Shredder and Voidcaller w/ demon. Keep Power Overwhelming to have a tool to deal with Innervate stuff.

Playstyle:

vs Combo / Ramp 
Probably my 2nd most hated match up. "Oh only 23 HP and can't kill my minions for a turn? Well you're dead." Combo Druid doesn't even bench, so go in hard and force him to deal with your minions with Force of Nature and co. You can often outrace him and burst him down in many cases. Once in the 9+ mana range, Loatheb can save you for a turn if you need to. Defender of Argus serves a similar purpose. Ironbeak Owl and the buffs can be used to get rid of Sludge Belcher, Ancient of War and Druid of the Claw to apply more pressure, if you are somewhat close to lethal.

Mulligan:

Ironbeak Owl is MVP in this match up. Perfect against Twilight Drake, Nerubian Egg and Voidcaller. Even two in hand is somewhat decent, they will targets. Otherwise, mulligan normal.

vs Zoo / Demon / Mid-Range / Mirror
Trade effectively. Hope your RNG is better than his RNG. Imp-losion and Imp Gang Boss are very strong in this match up. Sometimes it's advisable to go for face, since he will trade anyway, but only if he has no tokens. Always assume he has buff cards in hand. Killing Nerubian Egg and then using Power Overwhelming on your own Nerubian Egg to kill the Nerubian is a great way to swing the board in your favor.

vs Malygos
Same as above, but keep Loatheb for turn 9+, either to set up lethal or to prevent you from dying if he gets Malygos onto the board. He most likely will be forced to spend his damage spells to keep up with your board anyway.

vs Handlock
Ignore Twilight Drake if you can't deal with it effectively. The overarching tactic is to set up lethal and burst him down with Doomguard and Power Overwhelming. Loatheb can prevent Shadowflame shenanigans. Owl can deal with taunt if you have lethal and have not used it on Twilight Drake. Always assume he has Molten Giant and Sunfury Protector / Defender of Argus in his hand, so you should aim to get him close to 12-16 damage, depending on the mana level and your burst damage potential. So if he has next turn 5 mana available, you wan't his Molten Giant to cost 4+ mana, so he can't play it and Sunfury Protector at the same time, unless you have Owls or something to deal with taunts.