Arena is the real draw to the game, at least right now. Can you imagine how much fun a constructed MTG format would be if you could just play cards from a single core set? Yeah that's Hearthstone constructed. Given that this is Blizzard I am also concerned they will have a hard time with card design at a decent rate of release. At least Crypto had guys I knew and trusted making cards at a good clip (the last set they released for their WOWTCG game was the best one they made, and they got monotonically better in design over time in the stewardship of that game, even recovering it from a Urza Saga level of design incompetence from Upper Deck).
More I play this, the more I realize that this isn't the game for me. Will still play the dailies and arena but am much more interested in HEX's PAD-style event model and PvE design, and much more robust ruleset. Half of the time Hearthstone seems to take the "super simple" route instead of the "kind of simple, but leaves things open to crazier things in the long term" route that may be healthier for the game.