Unfortunately I'm still pretty unwell and haven't had chance to play much hearthstone BUT I have looked through the GvG cards so I'll give my initial thoughts here.
I'll start by saying that this expansion MAY vastly reduce the amount of zoo not only because it adds more tech cards to help decks fight zoo as well as more early game minions some decks may utilise to help against zoos early game, but also because demon warlock might a better deck with the new cards introduced. Looking through the cards I wouldn't say that zoo gained a huge amount either. Still, zoo will always be viable with its great snowball potential and ability to punish greedy decks and bad draws.
Ok first things first. The Soulfire change. This change is pretty huge honestly. That extra mana really hurts your potential for an early game tempo swing removing and playing minions, but doesn't remove it completely. You will often be able to fit 1 mana in to remove something, but you will have to consider this when trying to play on curve and mulliganing for a good curve. The biggest difference will be that this change may remove the ability to take initiative against other early game decks on turn 1 or 2. Honestly I will need to do some play testing to get a feel before thinking about removing Soulfire.
I have already gone ahead and made the most obvious change to the deck. Clockwork Gnome is clearly just a better version of Leper Gnome since the spare part it gives will likely hep you take control of the board. What you need to realise about playing aggressive decks that are minion based rather than burn decks like some hunter and mage variants, is that controlling the board translates into way more damage over time by keeping minions alive to hit face later, or soak up threats so OTHER minions can hit face later. Standard ZOO IS NOT ABOUT FACE DAMAGE rant over. Looking at the spare parts there are some pretty useful effects. Stat buffs are always nice. Freeze may help push damage, or delay a threat you are struggling to deal with helping you develope the board further. Stealth can help you protect valuable cards like Undertaker, Knife Juggler, etc, or just low health minions, or if you are desperate help you make a late game push for damage. Taunt can also help protect valuable minions, and works nicely with Nerubian Egg. Returning a minion is obviously great when combo-ed with buffing givers like Abusive Sergeant.
There's a couple of other changes too and I'll talk about those cards below. The deck will likely be changing a lot until the meta settles and the new cards are tested. Right now I'm thinking about how best to smooth the curve around the 3 and 4 drops. The problem is that I don't want to drop any of these 4's, but I'm not sure you can get away with a curve so clunky in zoo if you are looking for the consistent draws.
Now. Time to look at potential new cards for zoo!
Has potential in a mech deck, but is very weak alone as a turn 1 play which takes away a lot o the strength of him being 1 mana in the first place. Probably too situational.
This card is simply too low power for me. The protection and synergy with buffs in nice but I don't think it's enough when the 2 mana slot is already quite competitive.
This guy is the key to mech decks. It's the synergy card that makes playing mechs worth considering, the mech undertaker if you will, although without the same gamebreaking potential.
This card has some decent potential but it's low health and lack of initial impact means it probably isn't right for zoo.
I really like this card. It's just fun Harvest Golem right! But Harvest Golem is here to get a job done, not to be fun. Sadly this guy is 4 mana as well as less inconsistent, but even the worst case scenario of spawning something like an Owl is still fine. The 4 slot is really cramped for zoo with these new cards but I think the Shredder is a good addition.
Shieldbearer was a key card in pre naxx zoo and this guy does the job for 0 mana! Or does he? 4 health was a mountain to climb for your opponent in the early game, but 2? Easy. I probably prefer Shieldbearer over this guy in a non mech deck, and Shieldbearer isn't in the deck...
Look at that effect!!! Wait lets think about this rationally. 3/2 for 4 is pretty bad, and the taunt effect can actually hurt you if you taunt a valuable minion that you had taunted minions protecting, making it an easy target. So is it worth it? Yes. I like cards that are consistent. Cards that will get you win streaks, not just wins. But windfury and divine shields on your whole board? That's too good to pass up I think. Divine shields are SO good for early trading and keeping control of the board AS WELL as protecting minions from AoE that often starts cropping up around turn 4. Windfury is less effective when you have mostly weaker minions, but coupled with buffs and especially later on in the game it will allow you to push for a lot of damage.
So the logic here is going to be that once this card is in play you don't need the rest of your deck to win. Sadly I don't think it will be effective enough at closing out the game due to it's lack of initial impact, and the number of counters it has. Also these counter (Big Game Hunter, Shadow Word Death, etc) are likely to see more play with the number of big minions GvG brought in. When you consider that your opponent playing 3 cards next turn loses you 9 cards that's pretty bad, but when you consider that your opponent leaving it alive another turn and dropping another 3 cards loses you another 9... now you've just fatigued yourself for 8 damage. That's obviously one of the worst case scenarios, but I don't think the up side is high enough.
So what else does GvG have for us? Not much to be honest. Maybe some Goblins and Gno... what are Troggs?
A nice effect but the problem here is that the first spell this thing sees is likely to be the last. It's 4 mana big brother is sturdier but simply isn't a fit for zoo. But what about it's daddy?..
This is probably my new favourite card. Turn a damaged minion into a completely new one to 'heal' it. It also has a great synergy with buff cards like Abusive Sergeant and Enhance-o Mechano that create weak minions due to the cost of the buff. Sadly I don't think this card works all that well in zoo since you are already often picking out the best value cards at their mana cost.
This card looks great at first glance but don't be fooled, while he has a lot of potential he is no super Shattered Sun Cleric because he buffs minions played AFTER him not minions on the board. Why is this important? Firstly because that means the buff can't be used that same turn as a pseudo-charge, but more importantly because that means you either need to hold cards to combo, or hop Hobgoblin survives on the board, neither of which are appealing options for a zoo player. He is a perfect example of a card that needs play testing to really see not only how often he works out, but how effective it is when he does. Although it's decent value playing him with a Haunted Creeper turn 5 for example, that's a turn you spent building but not controlling the board, which can leave you open to a swing turn from your opponent. Also it's worth mentioning I believe he doesn't work with Imp-losion or Haunted Creepers deathrattle (unless of course you can pull the imps/spiders back to your hand and replay them).
In zoo there is such a thing as too big. The deck is designed to curve low so you aren't drawing high cost minions and having weak turns early on. So is 7 mana too big? Maybe not if the card is good enough. Ok... So is this card good enough? I guess the best comparison here is Loatheb, especially since that is probably the card it's competing with to get into the deck. It might look like Troggzor the Earthinator's effect is obviously superior, but actually it's not so clear. Loatheb has the ability to make it completely impossible for your opponent to use the spell he wants, leaving him a sitting duck next turn. That makes Loatheb much better if you're setting up for a kill. The extra stats on Troggzor are nice but don't get close to making up for the difference in mana, and that really is the problem. Troggzor might be good enough to justify the mana cost, but in zoo you often want your anti spell card a bit earlier, and if it doesn't get good value, for example facing Handlocks giants or big Druid minions, the card is useless for zoo. For now I definitely prefer Loatheb.
This card is not a terribly zoo like card. Although it does apply a lot of pressure to both the board and health of your opponent it's very expensive and doesn't have an immediate effect when dropped. So why am I including it? This card is op. Simple. It is probably the strongest card in the GvG set and right now you could probably put it in aaaany deck (as has been seen from most high level play). Why is it so good? Normally these big expensive minions suffer the draw back of puttng you behind if you play one and the opponent has an easy answer, but here even if Boom is answered, there are still the bombs, and between their attack and deathrattle, that is like having 4-10 power to play with almost. With luck they can clear big minions between them, small minions each, or just put a decent amount of damage onto your opponent. And that is the worst case scenario where your opponent DOES have an answer for Boom! Having to hold this till 7 will hurt, especially with doomguards desperate to be played before that, and drawing it early could lose you the game so I wouldn't actually recommend it for zoo, but I felt I needed to write something about it because it is so dominant in the meta at the moment. Oh and before I forget it buffs undertaker twice with the bombs which could be handy.
What about Warlock cards. Did Gol'dan get any new play things?
Warlocks class cards already seemed a bit weak in comparison to other classes, and with the Soulfire nerf I was hoping for maybe a bit more in this expansion. Demon lock got some shiny new toys, but zoo shouldn't complain. We are playing Warlock for the hero power not the cards. That being said there is one card here that is definitely worth a look. The problem is that for 4 mana there's a good chance you won't remove the minion you want, and you you'll spawn some imps ready for Swipe-ing. However, it is good value and works well with buff cards and REALLY well with Knife Juggler. The question is, is it good enough to get in the already hotly contested 4 mana slot?