Basically, Marvel's head of movies (Kevin Feige) came to Joss with the structure of the movie, and Joss then worked within that*, and it really, really shows. The basic story is dull beyond belief, repeated from millions of cookie-cutter comics. "Bad guys shows up, good guys fight amongst themselves, things get bad, good guys pull together, epic fight, good guys win." A plot so tired that it can fall asleep itself, probably joined by half of the audience.
What Joss does brilliantly is give character moments through the movie. The interaction of Tony Stark and Bruce Banner, Captain America's observations, Black Widow being generally awesome, and numerous things that I shan't specifically mention, but added up to a sheen of fun and depth placed over the overall structure of nothingness.
None of which makes it a bad movie. For an action movie with a non-existent plot it's great fun, and more deserving of cash than most other examples of that type. It just felt like it could have been more, and it's a shame it wasn't.
My favourite two reviews are this one over on Angry Flower, which points out many of the good things the script does, and Abigail Nussbaum's review at Strange Horizons, which also looks at the good and bad bits of the structure.
*It started out basically with Kevin (Feige) and Jeremy telling me “We know the basic structure of how they come together, what works, what doesn’t work, and how we see the climax,” which was nice, because he gave me a basic skeleton of three acts that I knew I had to hang on and then it was just a question of “How do I get there? How do I earn that? What moments would cause these people to be in that situation?” - from here
Original post on Dreamwidth - there are comments there.