I'll say this about AC1--it was a much better game in 2007 and 2008 than it is now. I was first attracted to the game because of the Crusades setting, which I still believe holds a wealth of potential that Ubisoft barely scratched the surface of (compared to how well they've mined the Renaissance for AC2 and ACB). Setting it during the Third Crusade was a stroke of genius and it was visually beautiful just a few years ago. This generation, including its sequels, has moved past it.
However, it was a redundant game for its time. There was no other game like it and the setting, as well as Altair himself, made most gamers finish it with mixed reviews, but it was still a compelling game. I'd still give it an 8/10 today. It's just that so many jumped on board with AC2 (the best in the series still, IMO) which lived up to the franchise's potential. That was a truly great game. Now, those who play that and then Brotherhood get around to the first non-Ezio game and are turned off after the first hour due to the repetition. While frustrating when the game first was released, the qualities of the controls, gameplay, story and visuals compensated. But with two games that vastly improve the gameplay already released, I cannot go back and play AC1 beyond maybe a few of my favorite assassinations (the big guy in Damascus, William de Montferrat in Acre and the archer guy on the boat in Acre). So, I understand the contempt newer gamers have for it.
I'll put it this way, AC1 was basically a demo of what the series could be and you had to be there to see why it was so great. Now we have the "real" games in AC2 and ACB. So much so, AC1 goes from being flawed, but very fun to being just a chore. Even so, I enjoyed that game so much in 2007 that I cannot join the bashing it now constantly receives. I think after AC3, Ubisoft should return to the Crusades at some point and try to do it better. Just my opinion.