Having an Avengers movie that focuses on Cap and his introduction might be very attractive to RDJ, EN and whoever they get to be Thor. It takes the pressure off them having to carry this franchise, along with their own. At the same time the identifiable characters are solely the link or key for the audience to relate to the movie.
I wouldn't think we would see too much character development of the established characters in the Avengers movie - they're there for the action. Development will happen in their own movies.
This might be a good reason to inject Cap here - to get the depth and breadth required.
In the end we have the trinity, but Cap has always been the main focus and leader of the group. This could allow for the introduction and some brief back story that could then be expanded upon in a solo movie (which I am hanging out for).
When you think off it, it's how they intro characters in the comics.