I thought Assassin's Creed was rather hit and miss. The plot was crap, it took itself way too seriously, and the combat was atrocious. Running across a city could be awesome, especially with soldiers chasing after you, but the controls were too hands off. Also, planning out and executing the assassinations could be awesome, but the death monologues completely knocked the goodness out of them. What ended undoing the game for me, however, was the repetitive and bland gameplay. If you've played one city, you've played them all and the side missions were completely uninspired collectathons.
In a sentence, the game's core idea was great but the execution was lacking.