I didn't go through everything yet but I don't think the bonus disc is disappointing at all. It has new extra features for all of the movies that were not on their original releases. In fact for Avengers in particular there's a bunch of extra footage they added in NOT found on the regular blu ray release. Again, i didnt go through all of the extra features yet but it seems like they added a lot in there.
As a matter of fact, do you guys remember, before The Avengers was released, how there was a scene of Captain America throwing his shield in an alleyway in some of the commercials and ads that never got released...that scene is included on there. I always wanted to know where that scene was from.