Assassin's Creed treads on new and interesting ground, with enough room for improvement to merit a sequel.
The combat, though fun at first, is forgettable, with the exception of the longer-lasting appeal of Altair's hidden blade. Enemies are numerous, and generally the main challenge is to avoid wave after wave of guards that the game throws at you, with two options available to you on how to take care of them: run, or kill.
I believe that my favorite part of Assassin's Creed was the idea of a Social Stealth game. The crowd reacts in a variety of ways, and while the boxart says they will either help or hinder you, it's almost always the latter. That's not a bad thing; it encourages the player to take on a stealthier role. If there is a sequel planned, I hope this aspect of the game has been much improved, if not adapted in other games as well.