Looking back on Assassin's Creed, I never got tired of certain game elements--especially the roof-top assassinations, but I found too way much of the game to be quite tedious: The battle sequences are especially uninteresting, and the sheer number of times I failed at simple tasks like pick-pocketing or just walking through a crowd unnoticed wore on me as the game progressed. Certain players will love that you can go back and collect all the flags, kill all the Templars, and complete all the side missions; it's very Crackdown-like in that way (as mentioned in the original review). There's just a lot of game here, though again, I'd argue that it's too long by far.
The end battles were especially horrible. The whole sequence leading up to the duel with Robert de Sable was even more emblematic of the tediousness of this title as anything else in the game, and the final battle at Masyaf--without ruining the ending--is, perhaps, the silliest example of repetitive game play I've ever seen. I'm glad it's over, and while I have a small desire to go back and collect some flags, the way they've implemented "free play" once you complete the campaign isn't particularly interesting to me.
Anyway, The end result is the same: Assassin's Creed could have and should have been great, but it ended up being pretty mediocre.