I can see your point about Marvel's superhero films embracing their comic book origins, but I think there are times when it hurts them. I thought the first Iron Man movie struck a brilliant balance between reality and fantasy, but felt the sequel suffered when it had to adjust its world to fit in with Marvel's other heroes. Where Iron Man made sure all the imaginative things going on in the film looked plausible, the second film made no such effort and the inclusion of Nick Fury and Black Widow distracted the story more than add to it.

In fact, I thought the way Marvel Studios build up The Avengers through Thor and Captain America hurt those films as well--more so in Captain America, because it felt rushed to include everything it could before setting up Steve to awake in the modern world. And a lot of potential character development for characters like Captain America in The Avengers didn't seem to happen because it would be handled in their own movies. The Avengers movie touched upon Steve not accepting his new world, but didn't do much with it, which is disappointing because Whedon emphasized how important his role would be in the film.

As for whether or not Robert Downey Jr improvised his lines, Marvel Comics Editor-in-Chief Axel Alonso said he heard Robert Downey Jr was improvising most of his script when talking about The Avengers. Here's the link:

http://www.comicbookresources.com/?page=article&id=38518

I am curious about how much of the Zak Penn's original script was re-written by Whedon. It will be interesting to see if there's any difference in The Avengers sequel if Whedon is given sole writing credit. Unless Marvel does the unthinkable and replaces Whedon like they did for the directors of Thor and Captain America.

I don't mean to sound harsh on The Avengers, so let me talk more on what I liked about the film. Like I said before, I loved Robert Downey Jr as Iron Man; he just fits the character so well. The moments between him and Bruce Banner were also good and made sense given their backgrounds. I also thought they nailed the Hulk--in writing and looks. And while I thought the film series played its cards too early by having a full scale alien invasion in the first film, it was fun to watch the Avengers take 'em on. And most surprisingly, everyone in the film were given a good amount of spotlight. No character felt cheated because of another. The post-post-credits was also funny.