As for other superheroes, well... This isn't really touched-upon by the Avengers series, so it could be that there ARE no other super heroes within this universe that were around to help. Whether the Avengers-verse stands on it's own, or the other heroes' stories are just off-screen. Presumably, Spiderman exists in their universe, since they have that in-canon spiderman cartoon on tv featuring Nick Fury and Coulson, but what the hell would Spiderman do to help The Avengers? He had no knowledge of what was happening with Loki, the Tesseract, and the aliens, making it above his super hero pay-grade so to speak. He would have no way of knowing what was causing it, all he would know is "Oh! A monster on a flying jet-ski! I should fight it and protect everyone in sight of me!". So maybe he was off-screen part of the story. In the end, its an Avengers movie, they were the center of it, so there was no need to cut away to Spiderman, even if Marvel studios COULD legally do that. As for the Fantastic Four, there's been many MANY times when they are "away on business" in other countries, other planets, even other dimensions, so its likely that they weren't home, or the FF simply don't exist as a super-team yet. As for the X-Men, they're a secret, they're fugitives, and they live in Upstate New York. Getting word of what was happening, getting the team assembled, suiting up, flying to Manhattan and getting INTO the city to help fight mysterious creatures they know nothing about, when they clearly see super-humans tangling with them already wouldn't be a good move. They would wait to see what happened, and if the Avengers DIDN'T stop them, of course they'd step up to help.