This movie was a lot of fun. So much could have gone wrong with this movie. it could have been a confusing mess. It is not for people unfamilar with any of the previous Marvel comics movies.
There was not too much comic relief. I still remember the second Transformers movie and those two robots. It was too much.
I like the term Terasect. Cosmic cube might be too seventies. Also Terasect works better with its mythic origins in the film.
For a while the second movie in a superhero series was significantly better than the first one. Examples include Spider-man 2 and the Dark Night and even X-men 2 to a lessor extent. I wonder how a sequel to Avengers could be significantly better.
In retrospect, the Black Widow’s part in Iron Man 2 makes more sense. For her role in the Avengers, she needed to be fairly established. To introduce her in Avengers might have slowed the movie down. To keep her out of Avengers would have required another female superhero to replace her. (An all male team would not be accepted.)
It might have been nice to have War Machine in the movie. Maybe it would have made the main cast a bit too large if he had been involved. (Hawkeye did not have much of a storyline as it was.) At least he could have been mentioned.