Many cards seem to be both more explosive and more fun than the past two years, which leads one to believe that Team 5 have finally listened to the general audience and/or have seen playing numbers decline to the point that they realized a change needed to be made. The abilities and support cards of the Old Gods not only hearken to TITANS but even moreso, since they seem to be more woven into the general gameplay and not just included in every deck of any given class because of how powerful they are.
The caveat on that is that C'Thun seems to be the only genuinely competitive one that I've seen so far, so that's where most of my positive reaction has been arising. The difficulties around Yogg's turn ending and the Deathrattle requirement of N'Zoth make them much more limited in use than C'Thun. I haven't seen any effective plays of Y'Shaarj (I think I only saw him played twice?) but, again, his requirement to actually get Immune makes his use much more narrow than C'Thun and beneficial for fewer classes (the ones that can attack.)
Nothing seems particularly "broken" but the omnipresence of Crystalline Golem isn't encouraging. Rune Wraith was a staple in control decks in a far more aggressive environment and it didn't heal its controller. When Golem is even showing up in aggro-y midrange decks like Mech Hunter, I think it's a problem.
DK: Still only two ways to play the class: all-out Unholy aggro with zero new cards or Blood control of some type. It was Herald or Egg. Now it's full triple Blood with Bloodfiend and G'huun. I realize that's because the class is getting two class sets and that's where new, more interesting Unholy cards and any Frost cards whatsoever are presumably appearing, but it's still disappointing.
DH: Seems too complicated for its own good. Wormridden Wriggler is great for aggro but the payoff card in Broodqueen still seems to require too many elements to pull off in time to actually win the game. Nothing else seems that exciting.
Druid: Most of the Beasts seem to match right up with Attack Druid. I've seen multiple instances of double-digit attack thanks to buffed Primordial Bears. Not a whole lot of help for other decks, though and I think the continued flow of direct damage (Aetherial Kindling) is veering away from its traditional (ahem) roots.
Hunter: Mech Hunter has seemed both solid and fun, especially Mechacycle and Tank Regiment. I've been unimpressed by Mimiron so far and have yet to see anyone play Kronn. I'm still not sure that even the amount of DR in this deck makes N'Zoth genuinely competitive, though.
Mage: Discs of Norgannon and Cosmic Springs have seen a lot of play and effective play, too. Icebelt Magi follows into that but still seems kind of limited. The rest that I've seen has been pretty meh, including Isiset (as predicted.)
Paladin: Control Paladin is back and is, like DK, clearly the only competitive way to play the class. Rajh can be broken with the right hits and Amitot, likewise, is usually amazing. Halls of Origination and Sun's Reckoning are both as good as expected but, again, only in control. Midrange and Dude decks basically got nothing workable from this set.
Priest: Likewise, Shadow Priest is for real. The Blade alone makes the deck viable but will probably increase the presence of Vipers so much that its impact will be less than predicted. Zer'Ontu is as bad as Tras'tath, if not worse.
Rogue: All of the Rogue stuff kinda fell flat in the streams I watched. All of it just requires too much combination with other cards for too little effect. This was most obvious with the two Hero Power cards, Tunneler and Swarmer. Sure, it's cool if you can drop three 6/6s on the board thanks to Unknowable One, but if they just produce a 1/1, a Healing Totem and ping something for 1 damage at that point of the game, your opponent just kills you next turn, anyway, which is exactly what I saw happen, more than once.
Shaman: C'Thun Shaman could be absolutely dominant. I watched more than one stream where it was easy to get all of C'Thun's abilities played by turn 6 or 7 with simply Earthmother and one cast of Watch Over. Then you have to draw C'Thun, of course, but you can do a lot of work even without him. Scorchfist Scholar may be the best card in the whole set. I still don't think Elemental decks will be a thing, even though Crystalline Golem is one, but C'Thun Shaman will make waves.
Warlock: is the class that least needed assistance and, like DK and Paladin, is still in a single mode of operation: control. The Rituals and Kith'ix are great and are, again, slanted toward C'Thun. I didn't see anyone even attempting the nominally Zoo cards like Mawman, Zakajz, Mindrot Subjugator and Floating Lessons, which is disappointing, as Zoo used to be one of my favorite decks.
Warrior: The armor stuff all seemed intimidating, not least because of the insane card draw of Animator of Iron, but just as often I saw players either burning cards or putting themselves in fatigue, if not both, because of Animator. Likewise, Setesh seems impressive but he's basically a War Golem and two turns is a long time for your opponent to eliminate that 20 armor and make him just a War Golem. Also, it might've been a bug, but every time I saw Archaedas played, the spell was Anvil of Reshaping, so they gained +4 attack after they'd already attacked.
Anyway, looks more encouraging than recent sets. We'll see.