According to this it's just the middle-of-the-credits scene, which I did see: http://aftercredits.com/2012/04/avengers-the-2012/

This was a better Hulk movie than both Hulk movies that exist, and I sorta liked those. I also didn't realize how much I wanted to see Hulk vs. Thor on film until now, but it turns out that the answer is "a lot".