The scene after the credits was the worst example of a fanboy moment EVER. I consider myself a pretty big comic book fan and I pick up on a lot of the Easter Eggs throughout these movies and they make me happy. That said, this would only mean something to 0.1% of the people watching the movie. Again, I like The Avengers, but this scene really meant nothing to me. Not like the post-credit scenes in Iron Man 1 and 2 or even X3.
I don't think the producers expect Joe Q. Filmgoer to know what the Cosmic Cube is. I'm not entirely sure that's what the surprise moment is supposed to be for that audience. I think the enticement is that Loki is still alive, and is up to something.
It still wasn't that well done, but still, I think Loki is supposed to be the big deal to most people, not the Cube.