You can probably divide Avengers fans into two basic categories: old-school comic book fans who were into the Avengers when they were just in the funnybooks, and new fans who discovered them (or at least became fans) once the movies hit.

That's the difference between a niche fanbase and a pop culture tsunami that followed the movies. The old-school fans had to make do with just the comics (which started in 1963), while the bigger fans took advantage of all the toys, models and gizmos that came down the pike after this movie hit theaters. That included things like Halloween costumes, maquette figures, and t-shirts, as well as cosplayers galore, fan fic, and art from young and old fans alike. (Source)

These goodies simply weren't around before the movie hit…and now even casual fans know who all these characters are.

And one of the coolest things about The Avengers is that it actually gave a little shout-out to all those fans in the closing minutes of the movie itself. We pan over a number of screens getting the man-on-the-street opinions of the gang that just saved the Earth, showing exuberant admirers sounding off about how cool their heroes are. (Source)

It's one thing for the fans to love this movie, but how many times have you seen a movie love the fans back like that?