All the warning signs are there: it's the 4th film, it's in 3D, virtually all of the original creative talent (aside from the actors) have moved on. Has the law of diminishing returns caught up with Shrek Forever After? Yes and no -- but mostly no.
Iron Man 2, entertaining as it is, and I like it, is a big missed opportunity. It could have been a lighter version of The Dark Knight, exploring deeper themes in a pop context with a more congenial cast. Instead, it's confused.