They stated the aliens were Chitari. The Chitari had their origin in the Ultimates. They are shape changerss (basically the Ultimate's version of Skrulls). They were in the Ultimates cartoon.
Originally Posted by rasx
Obviously, they did not use the shape-changing characteristics in the movie.
I know some people complained about them not being good villains - you don't know their motivation, they don't speak, blah, blah, blah. But that is a complaint you hear a lot in movies where the invading forces are aliens. To me, they are invading, they want your stuff, they are not going to stop and talk nice.
The real villain was Loki - and he was good at it.