All what Assassin's Creed 1 offered and more.
Story: I won't be ruining anything for you; however, the story picks up where AC I finished with; Desmond Miles in the laboratory, with a very exciting introduction. The story then changes to Ezio inside the Animus 2.0 and follows his story in the 1476 Italy.
Gameplay: Basically it has almost everything that was included in AC I: the familiar weapons, the blending with the crowd, the addictive buildings climbing and much more. In addition, it offers a finance system which can be used to buy weapons, armor, bags, items and even dyes. The missions are much more diverse and compelling, and the combat seems to take a more subtle sense. It also offers several collectibles that you can unlock several extra content with. Also there's this new Villa system which I can't speak about much since it's still pretty vague for me. One bad thing though: personally I found the fact that they changed the whole image of the assassins to be a bit annoying; Altair is far superior to Ezio.
Graphics: Needless to say; just like AC I the graphics are simply outstanding; the cities architectural design; the citizens and Ezio himself have very nice detailed graphics. Moreover, unlike any game I've played before, when Ezio is climbing, his hands and feet are always supported by something which adds a very nice sense of realism to the game.
Summary: Assassin's Creed II is a very nice and attractive game that you can spend hours and hours playing without getting bored of (the weakest point of AC 1 I believe). It has an intense story, amazing gameplay and a great value of bonus items which will keep you hooked for many hours of playtime.