People seem to neglect that around 95% of the audience of these movies have NEVER READ a comic book, much less a Marvel comic of Iron Man, Avengers, Captain America or Thor.  We are a small audience.  With that said, they can't introduce all of the nuances that make the characters great in a 2-2.5 hour movie.  There's way too much back story and history in all of these characters that we love.  It's 100% impossible for them to recreate it in movie form without having the large majority of the audience being completely lost.  "Why did that happen?  I don't get it!"  They need to dumb shit down for people.
Hell, look at Game of Thrones.  This show gets 13 hours (per book) to try and re-create the story, but you can't even do that perfectly for the sake of film.  There's entirely too much going on to capture everything.  I mean, how do you include thought bubbles for every character in a movie?  Sure, you could have 1 overlaying voice to create the movie from the perspective of a single person, but there's a lot more going on in a comic than they could realistically reproduce.
That said, I still didn't like IM3 that much.  Yes, it was entertaining.  I just didn't like that they made Super Paltrow.  It didn't make sense from a character stand point.  Oh well.