Assassin's Creed left most people disappointed. The ending was confusing and it wasn't the best game overall anyway. But Assassin's Creed II came and blew everyone away.
Assassin's Creed II has brought more knowlege into the present, involving Desmond using the Animus to train to be an Assassin, slowly involving Desmond into the story more and more. The gameplay is similar to the first Assassin's Creed, but is expanded a little, still being fluid as ever. The design is very good in the game, ecspecially the detail in Ezio, but not perfect. This game pulls you in a lot better, introducing a more open world and including fun collectables that will keep you playing even after you've finished.
The only bad thing I have found from this game is the small lack of design. I felt they could've tried a little harder to make the cities a teeny bit more beautiful.
All in all, a superb game.