Everything! New class, new act, ah removal, paragon 2.0, loot 2.0. These are all things we have been begging for since day 1! If all of the datamined stuff ends up in the game we will have the Diablo 3 that we have all been waiting so long for. Hopefully these awesome features will be announced at Blizzcon. Also can't wait for RoS Beta!
Everything surrounding loot improvements has me completely stoked for this expansion. I was absolutely not going to buy it, but when they announced that the Auction House was going bye-bye, I was sold, immediately.
I was super excited about the new loot affixes for legendary items. Some of them are just really really cool. I really just cant wait for loot2.0 in general as well as the removal of the Auction House. Should never have been part of the game.
The whole expansion looks really cool but the exciting news for me, is anything to do with the new crusader class.. The video of abilities looks amazing. Hearing more about it at blizzcon would be fantastic.