Let's keep this brief. I think Assassin's Creed is a poor game built on a great engine. Which when you tally up the averages leaves us with something very...average. Confused? Let me explain.
Ubisoft have nailed Altair. Nailed him. Nailed the game world, too, while we're at it. It's not often you get to play a game where your character is so responsive to your commands, and is so firmly attached to their surroundings. Altair doesn't float, he doesn't skip awkwardly between animations, he feels heavy, solid and real. It's a joy to simply run around a town, skipping over rooftops.
Shame, then, almost every other part of the game is either broken, rubbish or missing. Combat is rudimentary. Altair's voice-actor is awful. Missions are repetitive. The sci-fi plot is hammy as a room full of ham steaks. You get the idea. It's like Ubisoft spent all their time/effort on Altair and the beautiful game world, only to forget to work on the stuff you can do once you're actually in that game world. You know. The important stuff.
As a tech demo or a teaser for the franchise, it's wonderful. But as a game in its own right? With missions and variety and opportunity and all those gamey things? Not wonderful. Very average, in fact. Indeed the game's most lasting achievement might be that it laid the foundation for inevitable, superior sequels, where we get to keep the controls but also get hold of an involving quest or two while we're at it.