Quest druid looks pretty good and will have a ton of late game value. Will probably work quite well against control warrior, though it's early game is necessarily weak because of the quest. Unlikely to be more than tier 2 because of that.
Mage - Cyclone mage will continue to be powerful. Probably will remain a tier 1 deck, I don't see any big counters to it coming out.
Big spell mage I'm not sure on. Could be playable, probably will end up tier 3 because of a weakness to aggressive strategies.
Freeze Mage will continue to be an alternative to the cyclone mage if you want to do better against slower decks like control warrior. It didn't really get many new tools in this expansion though, but might be able to murge somewhat with the big spell mage so that you're playing big minions and big spells. Probably tier 2 either way.
Hunter - Mech hunter will continue to be around, but with no new cards will likely just be a fair bit weaker. It was already pretty mediocre and just played because it was simple, fast and easy to secure victories against bad players/poorly refined decks at lower ranks. With the expansion it will likely drop down to a tier 3 aggro deck no one wastes their time playing.
Quest Hunter initially looks pretty bad, but there are some pretty good cards printed for the quest and it's actually not as hard for them to summon 20 minions as it might initially seem. The hero power is also pretty insane if you can pair it with charge minions from unleash the hounds, leeroy jenkins, or with tundra rhino. Probably tier 3 but could make it into tier 2.
OTK Priest will be a tier 4 garbage meme deck and nothing more. If it was going to be good, people would already be playing Stormwind Knight + Power Word: Shield x2 + Divine Spirit x2 + Topsy Turvy for the same amount of mana and the same number of cards. It will die to aggro before it can get the combo off, it just hard loses to any sort of deck that has lots of taunts, control warrior will almost always have enough armor to survive it... I just don't see how this deck could possibly be competitively viable.
I can tell you've been listening to Trump ... or at least you make some of the same arguments he makes.
Quest Druid: What's its win condition? That was where I stumbled when theorycrafting it myself; it doesn't seem to have a good one.
Cyclone Mage: Well, there are big counters to it coming out, most prominently Plague of Murlocs. But I share your opinion that the counters won't be common enough to knock Cyclone Mage from Tier 1.
I'm not sure Big Spell Mage and Freeze Mage are different decks anymore. RDU's combination of the two looked pretty strong.
I've also seen the suggestion that Mech Hunter and Quest Hunter get combined into one ... Does adding a Magnetic minion to a minion already on board add to the Quest's counter? Anyone know?
OTK Priest: You made a very interesting argument; I had to think about it for a while. I'm ultimately not persuaded by it, though.
First of all, a combo using 1x Power Word: Shield is innately easier to pull off than a combo with 2x Power Word Shield, because it means you can use one of your Power Word: Shields before the combo turn, which is one of Priest's best sources of card draw.
I had the same question about the quest and the mech Hunter deck. The nice thing about it is all the deathrattle cards that summon multiple 1/1's. And the cards, like explodinator, that summon multiple minions for one card. Even if you finish the quest fairly late, a board of bombs via Flark that can attack sounds annoying to deal with at least, if not powerful. I dunno. Gonna have to find out though.
I had the same question about the quest and the mech Hunter deck. The nice thing about it is all the deathrattle cards that summon multiple 1/1's. And the cards, like explodinator, that summon multiple minions for one card. Even if you finish the quest fairly late, a board of bombs via Flark that can attack sounds annoying to deal with at least, if not powerful. I dunno. Gonna have to find out though.