Avengers wouldn't have worked out as well without the separate films. 1. Introducing each character: Avengers consists of 6 different characters from 6 VERY different universes. In the amount of 2.5 hours they would literally have to explain why there is a WW2 veteran transported into the future, a Green giant monster, a man in a robot suit, a Demi-god, a russian spy and Samuel L. Jackson in an eye patch are somehow connected in the same universe because you know... those things are kind of hard to understand out of context. 2. Back stories. Each one of these odd characters would have needed a back story. They would have to establish who Tony Stark is, his personality, his background. Bruce Banner's origin, his powers, why he is a fugitive. Cap's origin, why a man from the 40s is in the present, how did that happen, and etc. etc. etc. 3. Establish a main story. The plot has to connect each character in some way. 4. Threat. Have some kind of threat or threats. The villain, again has to be connected somehow to each of the characters, those character's back stories, their separate universes, etc. 5. Character development. Each of the characters in addition to showing their back stories have to grow as characters and go through some kind of arc together. If Joss Whedon would be able to pull all that off in a 2.5 hour movie. Then he would be a god. EDIT: It has nothing to do with how much each separate movie made in case that's what you think I was referring to.