A couple reasons off the top of my head why I didn't like the Avengers movie:
The Loki guy looked a little too much like an angry Pee Wee Herman. Plus I never quite followed the logic of what he was doing, why he allowed himself to get captured, what the deal was with the eyeball-scan thing he did on the guy, or how he even got to earth in the first place.
The Black Widow and Hawkeye characters served no purpose for the film. The film would have been completely workable had their characters not even been there in the first place. Compared to Hulk and Thor, they were pathetic in terms of what they brought to the team. Ooooh, arrows!
What exactly does Nick Fury do? And where did he vanish to during huge segments of time in the film? Why didn't he show up at the NYC fight to help out or something, perhaps with other agents of SHIELD?
The Avengers had it's moments, but otherwise didn't really work for me.