The movie just wasn't as fun as the first. The villains were weak. Sam Rockwell never found his comic groove and just came off painfully awkward. Mickey Rourke was just dull and looked like he didn't want to be there and felt like he was above it all. Downey Jr. worked his butt off to be charming and it worked for the most part, but aside from Paltrow he had no good foils to work off of. He and Howard had better raport in the original than did he and Cheadle.
Movies like this are all about expectations. What are you realistically expecting it to be? If you go in expecting it to be the best of the series, you won't get that. And it never could have been that way. The first will always be best because it was fresh and new and we'd never quite had an experience like that before. Sequels can only every be a replica of that experience minus the uniqueness. That's what Spielberg and Co. give us here. More of the same with some fresh twists here and there.