The Chitauri are from the Marvel Ultimate Universe, not the mainline Marvel. They went with them as a separate race (As opposed to darker and edgier Skrulls) partly because of some licencing issues and partly because, as Whedon noted, the movie had enough big names as it was and throwing in something like the Skrulls or the Kree wouldn't have done either race justice.
I saw it yesterday, and loved it.
I saw it yesterday, and loved it.