Only just started a couple of days ago. My thoughts?
First of all, loved Warcraft 1 and 2, they lost me with 3, but vanilla WoW was one of the greatest games I ever played. Been gone from Blizzard until the hugely disappointing Diablo 3. And I've never played any card-based games before, other than a bit of Pokemon as a teen.
But this is a hell of a game. So easy to pick up and play, so tactile, so polished. Only takes an hour or so before you see all the types of tactical strats that will become available. And they've done an amazing job of really properly representing what each WoW class is like with the basic decks for each hero. As a Warlock, the push and pull of summoning and sacrificing is just fantastic, and your plain-jane mage nukes everything just like a mage should. Those were my classes, so I can't really speak for the others. I love all the options a lock has to get more and more cards in your hands, that's what made that class fun as hell to me.
The way the card collection and deck building is set up is amazing too. Beat all the practice dummies, then moved to Ranked with a custom Warlock deck and won my first five. It was fantastic. The arena was next, and again it has a brilliant risk and reward factor. Just hugely impressed with it all. Until...
The grind. Sorry, but once I hit the treadmill I've had to decide to get off. 10 coins for 3 wins? If you're an average player that means 60 matches until you can buy a pack. That's more than 10 matches per new card. That's ridiculous, and I don't have that kind of time. At the real-money cost per pack there's also no way I'm paying, even if I'd be willing to toss Blizzard 15-20 euro for making such a great game. And to continue having fun I really need some new Warlock cards to make the kind of decks I'd like to.
So in summary:
Wonderful game, had tons of fun, but I'm not willing to invest the kind of time they seem to think I should have to.
Also, of all the classes I've played Mages are the only ones that seem a bit OP. Burst damage with plenty of removal potential. You play tons of low minions and Arcane Explosion and Missiles take them out. You play bigger minions and Polymorph and Fireball will deal with them. You control the board late game and Flamestrike turns the tables. Can't deal with mages at all. The rest I found either balanced or underpowered. I would be interested to hear what the early community thoughts are on the classes, as I deliberately looked at nothing beforehand and during playing the game so I'd have to come up with my own strats.