Having played a good hour or so of the game (and having previously owned the first game), my comments are as follows:
1) The world is as incredible as in the first, graphically and interactively.
2) There is a much more "in-depth" combat system. I disagree with the "1 button mashing" remark of earlier, as you find when fighting against the likes of Crusaders you need to make use of blocks and evades to fight effectively. AC2 features mobs that can be "recruited". You target a group of whores, thieves, street thugs, etc etc and pay a set fee to have them follow you to help you "blend" or to go off and attack/distract guards. It's all been improved, right from "throwing coins" to gather the common people to get in the way of guards, to being able to pick up and move/hide dead bodies (depending on what you want to do).
3) Everything is more "Personal". -You- are your own assassin. Yes, you have the role of Ezio to play, but you make him into what you want him to be. You buy health potions, new armor, new weapons etc etc...
4) "Notoriety" has been improved. If you're a notorious killer and commit public acts of indecency often, "wanted posters" are put up, and the local speakers will talk of your sins. This in turn affects a guards behaviour towards you, so you can tear them down to reduce notoriety.
5) You have a "stronghold", which, although I've not got to yet, I believe you can build up in a way that you want to... It has a "value" apparantely, and you can buy paintings and things for it.. So I assume it's a bit like a families House like in the Godfather series of games.
It's a much more "interesting" game than the first.. And there has been a lot less "in and out" like in the first game. The back and forth between Desmond and Altair annoyed me immensely, but so far, I've only had one instance of Desmond being active, and the rest has been as Ezio. The story so far has been top notch and I see only signs of it getting better.
EDIT: Now you can swim!! How could I forget that. EASILY one of the best changes they've made to the game. I hated dying because Altair "couldnt swim".