I think that the problem is that, if you have characters that can't carry their own movie, you have to make time in the Avengers movie for their origin story. It's not just to show how they got their powers; obviously, you could just have Hawkeye walk in and say "I'm the best archer in the world" and leave it at that.
You can work their powers into the movie, easily. Introduce Pym as a scientist working on shrinking/growth technology, maybe have Janet be the first human test subject or something.
They're already throwing in Fury without a movie explaining who he is, why not?