With all due respect, it is obvious you've never read a comic book your entire life. I believe the movie wasn't for people of your taste. this article should enlighten you to how deep the characters can be: http://birthmoviesdeath.com/2015/05/12/earths-mightiest-monsters-the-character-arcs-of-avengers-age-of-ultron
The characters may not be LOTR-Character-Back-Story-Deep but they do have arcs. Interesting arcs if I may say so myself.