Joss Whedon on the dysfunctional Avengers

Joss Whedon on Comic Books, Abusing Language and the Joys of Genre:
"They don’t belong together. They all belong alone. The more they’re alone, the less useful they are. And there are a lot of elements of that that got thrown out at script stage, in editing stage. A lot of slightly darker things. But the one thing that did stay in there was the assertion that there are people who can’t be controlled, and we need as a human race to deal with it. We need something to stand up for us. We either need to fight them or we need to make them fight for us. And it doesn’t matter if they’re just kind of skillful or they’re the fricking Hulk. What matters is they’re all people, and they’re all dysfunctional on some level no matter how cool they are. And the dysfunction comes from their inability to work in a group."
Look at it this way. The Fantastic Four are a superhero team who at least have pretensions of being a team. they wear colour-coordinated costumes and are even themed on the four elements -- Mister Fantastic is water (fluid), Invisible Girl is air, the Thing is Earth, and the Human torch is Fire. Contrast this with the Avengers who have no theme running through them. They just don't belong together.