I don't feel any of the movies were tied down because of Avengers.. the only connections were some Shield scenes in the aftermath and maybe post credits scenes and those were self contained and had no relation to the main movie plot.
"Tied down" may not be the right term, but it has felt to me like the studio was trying a little too hard to make all these characters fit into the exact same world and style, when they may not have been what's best for each individual movie.
Maybe Incredible Hulk could have been a lot more gritty, or Thor a lot more poetic and fantastical. I just think it would have been nice to see one of these directors do something as fresh and different with their
movie that Nolan was able to do with the Batman movies.
Instead of just making something as safe and audience-friendly as they possibly could each time.
I think that's more of a choice of the director what style and focus his movie should have.. i found Norton's Hulk very entertaining but really didn't like Thor that much because several elements just didn't click for me but i don't think the connection to the Marvel universe was the reason for it.
It is the state of affairs with movies these days that business comes first and movie studios will listen to their numbers guys more often than they will to the creative staff and if that creative staff tries to something unusual, i.e. a bold creative choice that might alienate customers then the studio will either nudge the director to a different way if they are curteous or downright put their foot down as in "This is how it's gonna be.."
Very few directors have the clout to stand up to such pressure and most superhero movie directors are somewhere in the middle.. recognized names but no Spielbergs or Scorseses. So we get safe movies that sometimes turn out better than hoped (Iron Man) or are not so good (Thor.. my opinion).
Captain America is quite good.. not as good as i hoped it would be but that's due to certain elements i disliked (see my review in this thread) but it could have been a lot worse so i'm content. It was a safe movie but it was done well and it had a bit of heart in it.