The Avengers was revolutionary in comic book history as it was Marvel's response to DC's best-selling debut of the Justice League three years before. This is the point, though - in three years Stan Lee had successfully built up five popular characters with fleshed out backgrounds and adventures. When all of them were brought together, we had a good understanding of each and every one.
When Disney was setting up this universe, there was this idea that, like Iron Man, each character would get at least two films to give them a little more background and character stability. Instead, Thor was diminished from a character rich with Norse mythology to a Shakespearean archetype, Captain America's history in the golden age of World War II-themed comic books was reduced to an underdog story cliche, and Iron Man's character development was deferred in favor of giving Nick Fury and Black Widow added screentime.