The after-credits 'scene' is not just a scene, it's the actual teaser trailer for The Avengers! Here is 'KapoMonkey':
The breakdown of the after-credits scene from what I can remember:
Steve Rogers is in an empty gym throwing haymakers at a punching bag before finally punching it so hard it breaks off it's chains and (I believe) through the wall. Nick Fury walks in and says something along the lines of "getting ready for the enemy?" and Steve goes "Well it's been awhile
*I don't know word for word exactly what they said. I saw the scene around 8 and I'm writing this at 1:30a.m. So yea I'm a little tired.
Then we see quick snippets of Tony Stark and Steve Rogers, Loki being escorted in "handcuffs" by SHIELD. After that it's a long shot of Thor rocking a new outfit reminiscent of his Ultimate garb (sleeveless) in front of some roundtable of SHIELD.
Then basically after that it's shots of action scenes with each character - Iron Man in what still looks to be his Mark 7 outfit getting ready to fire his repulsor beams, Cap in some sorta battlefield, Black Widow doing some flips and throwing darts, Hawkeye shooting an arrow, Thor summoning lightening, a shot of Bruce Banner taking off his glasses.
Then the trailer closes off with really quick action shots. Couldn't tell what they were, no actual clue as to who the villain(s) is/as we only see Loki. You don't really see a shot of all of them together though I could be wrong about that and missed it. Can't wait to see it hit the web and get some screen grabs.