Curse of Naxxramas, the first content expansion pack for Hearthstone, dropped last week. They're releasing it over five weeks, so it's not all out yet, but the Arachnid Quarter is out (and free), and the Plague Quarter opened up today (for 700 gold).
Each quarter gives three decently challenging AIs, beating each of which grants a pair of cards for the set. Beating all three grants a legendary card (identified as the 3rd boss). Beating all three also opens up two (so far) class challenges, by which one may earn a pair of class-specific cards.
However, the real addiction comes from the fact that beating all three bosses of a wing on normal unlocks them all on Heroic, which is a super-difficult setting. I'm still trying to beat Maexxna on it! (As an aside: Counter-intuitively, I found Druid to be the easiest class for defeating Faerlina on Heroic.)
Must... win... Heroic!...