Hearthpwn

Chameleos - really good card vs almost any deck. But if you're not sure if you want to craft it, you can replace it with Stonehill Defender - you'll discover card you need in exact situation + some better fight vs aggro decks.
Also you can take Curious Glimmerroot - as for my experience, you will almost always guess opponent card correctly (assuming it sometimes shows odd cards for even decks :D), but no one guarantee this card won't be useless for you (with chameleos you can choose whether to pick exact card or not).

Twilight Acolyte - main purpose of this card is shutting down Cubelock's 8/8 giant on turn 4 (that is why you should keep Acolyte vs warlock and DO NOT play it on turn 3 if opponent is keeping cards). Closest replacement would be Big Game Hunter - however, it's expensive too.
If you don't have Acolyte, you can still kill giant with any 3-attack minion set on turn 3 + Scaleworm on 4th, you'll just need somewhat more luck to get both cards in time. So replacement for acolyte would be any 3-attack (3) minion like Curious Glimmerroot

Lady in White - can't be replaced. Altough deck is still playable without a Lady, your lategame vs control decks will be much worse.

Prince Keleseth - same as for White Lady, can't be replaced. Altough deck is still playable without Prince, your gameplay would be worse. Only positive shift could be that you'll be able to add some (2) minions, like Squashling or Acidic Swamp Ooze

Gluttonous Ooze - you can replace it with Harrison Jones or Corrosive Sludge. This replacement won't hurt your deck too much.
Versus most current meta decks you don't need more than one weapon removal.

Nightmare Amalgam - really good card, it's 3/4 Dragon for 3 mana. You can replace it either with Dragons or with 3-mana minions (like i've mentioned above).
Candidates are: Stonehill Defender, Glimmerroot, Mindbreaker etc.

Mossy Horror - through it's unique ability, this card can't be replaced. Second Spellbreaker, may be ...

Skulking Geist - Must Have card vs Taunt Druid and useful against Cubelocks and odd Warriors, but you can play without it. Anyway, Taunt Druids became really rare nowadays :-)

Spiteful Summoner - come on, guys :D

Primordial Drake - any other big drake (Ysera, Deathwing, etc) would be fine. I would even add Nozdormu if it would cost 8 mana :-)

 

You should always keep Keleseth, Chameleos and Cleric vs any class (there could be exception for Cleric i'll write below)

Warlock:
Mulligan:
Almost all warlocks you face would be cubelocks. Against their slow play you can keep Spiteful Summoner, Skulking Geist and even Lady in White. Sure you should keep Gluttonous Ooze (or Harrison/whatever) and never play it until Skull is used.

Guide:
Only early game threat you should care about is 8/8 giant on turn 4. There are 3 ways to get rid of it:
1) Any 3-attack minion + Scaleworm
2) Twilight Acolyte (You should NEVER play it on turn 3). Problem with Acolyte way is that it leaves 2/8 giant (which can be cubed into 2x 8/8), but you also get 8/4 minion for yourself
3) Mindbreaker (or any card spent by warlock before turn 4) delay giant to turn 5 or later.

Other threats are: 
Skull (5 mana) - totally countered by Ooze/Harrison.
Voidlord (if played on turn 9+) - Mind Control (smartest of cubelocks even use dark pact on their own Voidlord). Mind controlled Voidlord is not resurected with Gul'dan ;-)
Whole board of 1/3 Voidwalkers - Mossy Horror or Duskbreaker. Your side of board don't suffer much with both ways.
Dark Pact to heal and break Cubes/Lackeys - Skulking Geist
Defile - try not to leave much low health minions or, at least, not to leave good "ladder" (kill your lowest minions, heal those in the middle.
As for Lord Godfrey - you can't really play around it, relax :-) Cubelock will take out your whole board at least once, so make sure you have some cards in hand (Do NOT put too much on board when you're ahead, 2-3 big minions are usually enough)

Last, but not the least, is Rin, the First Disciple.
1) First of all, if you played your Skulking Geist, warlock have no easy way to kill Rin himself - you will have time to MC or silence it (beware, though, warlock can have extra Rin from Stonehill Defender)
2) If he's get Rin killed and got seals in his hand, you usually can make enough pressure to don't let him play it (beware - Rin locks usually have 3 total removals: 2x Twisting Nether and Lord Godfrey - try to not use too much cards even if you have mana to put extra minions (keeping 2-3 minions on board is usually enough to make him defend, not rolling seals)
3) Never use chameleos to steal Rin herself, but you can copy seals (you'll have tempo advantage) or even wait for Azari himself. 
For my experience, burning warlock's deck is usually senseless, cos he usually have little cards left at the moment Azari can be played. By that time your troops, inspired by Lady in White, can usually finish warlock's face.

Paladin:
Most of the time you'll face Even or Murloc paladin (and Odd from time to time). Mulligan is the same against any of them, however.

Mulligan:
You should look for Tar Creeper and Duskbreaker. You can also keep Mindbreaker and Nightmare Amalgam, especially if you have 1 or 2 drop. Mossy creeper is great too, but main part is to stay alive to turn 6 :-)

Guide:
Play whatever 1-2-3 mana minions you can scrap and pray you get Duskbreaker to turn 4 (if not, you're most likely dead). However,Scaleworm can be good enough too.
You should clear board as much as possible (even if that means killing 1/1 recruit witn 5/5), and should go face ONLY in two cases: opponent have no minions OR you can kill him right away.
Even paladins can kill any board with Equality + Consecration/Avenging Wrath so don't rely too much on your buffed Wyrmguard [/card] :-) and trade, trade, trade as paranoid with every minion they have on board.

Try to save second [card]Duskbreaker in later game to clear Sunkeeper Tarim's fallout (use Mossy creeper and Primordial Drake instead, if possible).

Druid: 
Most of the Druids are Spiteful (some of them with aggro opening), but you can face Taunt Dru from time to time.

Mulligan:
If you have any 1-2-3 play or a Duskbreaker, you can keep Spiteful Summoner, although it can turn out too greedy. You should also keep Twilight Acolyte to use against spiteful's summon.
You can also keep Skulking Geist (unless you early play is too bad) for the chance your opponent is Taunt one (if it turns out Taunt Dru, you have only 12-15 turns to play out Geist, else you're dead)

Guide:
Incoming :-)

Warrior:
Most of the time you'll face very passive Odd and/or Quest warriors, so you can be greedy with keeping some lategame cards in your hand.

Mulligan:
Key cards you need are Mindbreaker (NOT to play on turn 3 as you have only one in deck) and Skulking Geist, you should also keep Spiteful Summoner or even Lady in White.

Guide:
Incoming :-)

Rogue:
Hardest class for our deck: there are Quest and Odd rogues in meta and you should prepare for Odd (no point to prepare for Quest rogue bcos you're dead anyway, unless you get really lucky :-))

Mulligan:
You should keep Tar Creeper, Gluttonous Ooze, Scaleworm (to kill Hench-Clan Thug or Vicious Fledgling) and Duskbreaker (to clean board before Fungalmancer come). Good cards are also Mossy creeper and Mindbreaker, you can keep them if you already have good 3-4 turn plays

Hunter:
There were some odd hunters before, but most of them are secret now (easy matchup for the deck)

Mulligan:
Keep early game minions, Gluttonous Ooze and Mindbreaker (try to draw out at least one Kill Command before putting it) and use Duskbreaker ONLY to clear bunch of 3/3 wolves.
If you happen to face Odd hunter - try to draw out every possible removal and put a taunt before playing Mindbreaker - this card can really save your ass.

Shaman:
Haven't seen Shams for a while. Shutterwocks are dead and even sham didn't impress me much. 

Mage:
There are aggro mage and control one in meta - we will prepare for aggro one as most common (and more dangerous on early game):
You can toss out Cleric (especially if you have Keleseth for turn 2) - mages regularly play 3/2 girl who can spoil all the fun for Cleric). You need to find Tar Creeper,Duskbreaker, but can also keep any other 3 and 4 drop.
If you have any other 1-2-3 plays, keep Gluttonous Ooze, you'll need it against Aluneth (much will depend on will you be able to shut it down fast). 

If you happen to face control mage - good luck, it's really hard match-up for the deck. You'll need to play around plenty of his removals and, as a win condition, steal Frost Lich Jaina with Chameleos :-)

Priest:
Most of the time you'll face control priest with Dark Anduin. You don't really need any early game plays, just focus on late: Spiteful Summoner and Lady in White. You also would  need Mindbreaker (DO NOT to play on turn 3 as you have only one in deck) and Skulking Geist (deck killer if you face Combo Priest with inner fire :-))

First game (vs priest):
On mulligan i'd toss out 2 of 3 of your (3)-drops. Keeping Glimmerroot is ok

Second game (vs paladin):
On mulligan i would certainly toss Lady and may be even Mossy guy, but keep Mindbreaker (nice turn 3 play vs both even & odd pali). Main goal playing vs paladin is to survive first 5-6 turns, and you don't really need Lady to win lategame :-) 
Usually you have to find Duskbreaker till turn 4, else you're much likely dead. 

Third game (vs Warlock):
You should NOT trade keleseth with 2/1 cobold (kill it with worm instead) bcos you'll need that extra 2 damage for giant (which occasionally came right next turn).
Afterwards i'd kill weakened giant (just in case he have Cube + dark pact) . You don't really need to hurry with killing Cubelock right away, bcos unlike many decks, this one counter Cubelock on all the stages, on lategame as well - make trades if thats make board safer and give him no possible chance :-)