I always said that Assassin's Creed 2 was one of those games that really needed and deserved a sequel.
Assassin's Creed was at it's core a really really good game. The flaws were not in the concept but rather in the half-arsed execution. Some things didn't make sense and the missions were overall pretty repetitive, but the controls worked awesomely, the graphics were beautiful, the environment was great and the story was good enough. It was easy to point out the flaws and I can imagine it's rather easy for the developers to take note of and fix. Now that the second one promises to fix the repetition issues and add more weapons to make combat more interesting, we'll hopefully have a really good game instead of "just" a really good concept.
Besides, the setting couldn't be more awesome. Anybody who's been to Venice knows it's kick ass, can't wait to sneak around in it in AC ^^