Blizzard also added the Heroic Challenge, where difficulty instantly ramps up. For example, Anub'Rekhan doesn't summon a 3/1 for 2 mana anymore, no he calls for a 4/4 for the same amount of mana. Similarly, Maexxna's ability returns 2 random creatures in hand, and costs nothing. To ramp things up properly, the bosses have a 45-point health.
There is some challenge towards 'em, quite a lot, since some decks work exceptionally well, whilst others don't even make a dent in their health-pool.
I feel that it's somewhat frustrating to clear all wings in Heroic Mode, and all I get is a new, exclusive, albeit good looking, card back.
Overall, the expansion is a good start. It's somewhat short, but if you're a Hearthstone fan, you will get a lot from it (especially since it doesn't cost anything), and you be waiting for the next Wing. I am certain I am not the first one to notice this, but Blizzard is like the perfect Dealer. The first batch is for free, enough to get a taste, but everything else, well you gots to pay to get the kick.
Of course, that doesn't mean I will veer away from any future wings, not even by a longshot.