Personally, I'm looking forward to the continuation of the storyline, though I'd say my second most anticipated feature is the multiplayer.
I hope they really fix it this time around, as it seems that ACR MP had some good things added, but the majority of it was poorly implemented, and I would like it if the characters stood out more. (no more pick and mix customization and copy and paste animations)
I am impressed and basically looking forward to everything i have heard about AC3.
This pretty much.
-New gameplay mechanics, features, so on
-New graphics, animations/ techology
-Little details and everything we've heard so far
-Hutning, new weather and season system
The storyline. Definitely. I'm looking forward mostly to the modern day story and the conclusion of the whole "cycle" as the developers like to call it. Also, the notorious WTF ending we've already got used to, that's a must.
I don't really care about the historical setting in this case. But I'm looking forward to meeting Connor. From what I've seen of him so far, I think I'm going to like him (maybe as much as Altair and definitely more than Ezio).