It's really interesting to see the design decisions made by the Hearthstone team and contrast them with Magic. Obviously the most fundamental difference is the turn structure; in Magic, you can't do anything unless your opponent says "okay" (sure, they say "okay" 95% of the time), but in Hearthstone you can't do anything at all to interact on your opponent's turn. This has a strong side effect on the mana system; I can't manipulate play order to gain a mana advantage by using it on your turn. Couple that with the fact that you can't (certain keywords notwithstanding) stop your opponent from just choosing to ignore your creatures, and the game ends up with a lot of steamrolls. Once you get going, it's hard to not stay ahead.
I get why Hearthstone did this - allowing interaction on your opponent's turn slows the game down. But I really think it's holding the game back by cutting down on amount of design space they have and the amount of play skill allowed. This is of course my assessment after playing for just a few hours, so take it with a grain of salt. And I'll admit that I wasn't particularly expecting to like it, so perhaps I was carrying some bias in. But, honestly, I don't see myself spending a lot of time with it.
That being said - Wizards could learn a lot of lessons about presentation and user experience from this thing. It's gorgeous, and the game play is silky smooth. Magic honestly deserves an online play experience like that.