It's just natural to compare a sequel to the original movie on which it is based. Iron Man, in 2008, was a hit for several reasons, but mostly for the amazing on-screen presence of Robert Downey, Jr. His Tony Stark was confident, sexy, arrogant and funny--and we loved it! The audience was surprised a comic book movie about a guy in clunky body armor could be so well made, and so enjoyable to watch.
Iron Man transcended the comic book movie genre.
Iron Man 2 fits right in.
The sequel feels a little full of itself, as if it's still riding the success of Iron Man. The story, which is pretty forgettable, takes too many shortcuts and too much liberty with the characters to pull off a solid production. Watching Stark fight for his life in the first flick is compelling; seeing him gorging on donuts during an existential funk is simply pretentious.
Iron Man 2 has terrific action and a great deal of drama. But there is little foundation on which to build the story. It's a comic book movie, after all.