If I had to pick one thing I would go with the class changes. But really its everything together that makes me excited for the future of D3. Any one thing isn't much but all together the game is evolving so much at once!
The new loot system is the most exciting! My cousin plays the console version of D3 a lot and if the new loot system is like that it's going to be awesome! I know some will call it welfare epics but I love PvP and being able to get the gear to do top rank pvp much easier will be awesome.
Cinematic had me drooling. But most excited for the new class. From what I can see it will use a shield? As of now shields are nearly obsolete and I've always loved playing a sword and board character!