Basically... Marvel is treating their movies like their comic books, thus giving them the same creative problems their comics have.
Most comic fans just care how "cooool!" it is to see Nick Fury or Captain America's shield than whether they really help the movie or not. The movie did nothing to explain who the F Sam Jackson really was or who these Avengers people were or what they have to do with anything really. It all just exists to feed the next franchise.
Thank god Christopher Nolan won't have to deal with this sort of thing for Batman 3 (...we hope)!