Constantinople. They added a whole new mechanic with the hook-blade, and you can now craft your own bombs and there's a good number of types. Oh, and now they have a "Defend your den" mechanic, which is a lot like an Assassin's Creed version of Tower Defense or something You know, those little internet games where you have enemies going along a path and you have to keep them from getting to your base? It's not that simplistic, of course, but that's the idea. Point is, there is some new stuff

They also added a whole little mini-game section where you're going through a null-programmed area of the Animus, learning about Desmond's past. It's not very fun game-play wise (very straightforward and simple), but it's interesting to get the background.

I'm not very far into it, so I can't really tell you too much more. I do think it's worth playing EVENTUALLY, at least