I'm in the beta for Hearthstone. Overall, the game is pretty simple to learn. The mechanics are similar to Magic, and those that aren't are easy to accommodate to. The card pool is pretty small, but large enough to have at least some variety among different decks. So far, there are at least three "tribes" in way of creatures; Murlocs, Pirates, and Beasts. I hesitate to include Beasts, as they are majorly Hunter specific, but the other two are class neutral. Class balance is still a little bit off, and every now and then you'll see a trend of one class pulling ahead in terms of dominance and overall performance.
Grinding for cards is at times tedious, as there aren't enough ways to earn enough gold to buy a pack a day, unless you grind games heavily and have a high win ratio. Heading in to arena can prove to be a great, or horrible experience, as card pools seem to lean towards one extreme or another at the moment(from my experience). You either get a very good card pool for your class, or you get a terrible pool for your class that another class would fare much better with. However, regardless of how you place, you will be rewarded, and for that I find it encouraging, and don't see any reason to use the gold for anything but rolling around in the arena. It feels a bit like doing a booster draft each time I pay up for it, which is helpful either way I look at it.
Overall, I'm impressed with the game. For a small production team, they've created a pretty solid piece of software that is pretty enjoyable. the only thing I'm wishing is that they would give in and incorporate chat rooms and/or a private message system that can be used out of game to communicate with people you have just played with. I have had some amazing games on Hearthstone, and it bothers me that all I can do is toss out a "well played" emote.
I'm looking forward to seeing how they handle content post release, and seeing if they can keep up with the demand for refinement and improvement. Producing new "sets" of cards will be one of the harder things for the Hearthstone team to do. I'm also hoping that they decide to release a mobile game, as that would attract a massive crowd. I know that if it were available for a mobile device, I would be sure to play it whenever I had downtime, as it's easily the most entertaining option out there for a possible mobile game.