I walked to Roosevelt Field to watch a matinee of Iron Man 2 yesterday. While I hesitate to compare it to the first movie on the whole, I will mention elements I thought were better, equal, and worse.
Don Cheadle is a better, more confrontational James Rhodes than Terrence Howard, who I felt played the role too softly.
Mickey Rourke's character Whiplash/Blacklash was on par with Jeff Bridges' Obadiah Stane. I thought Stark's business rival Justin Hammer (Sam Rockwell) was naive to think he could control Whiplash, but I suppose powerful people do get used to things going their way.
Robert Downey Jr. still plays Stark so naturally, and best of all were the internal conflicts Stark faced: the miniature reactor in his chest rapidly killing him, the strain caused by keeping this a secret from Pepper, Happy, and Rhodey, Black Widow operating undercover at Stark Enterprises. While many sequels have flown out of control, these human problems kept Iron Man 2 grounded.
Finally, I thought the soundtrack to Iron Man was excellent. None of Iron Man 2's songs jumped out at me.
Marvel's unique attempt at continuity between their superhero movies is working. Iron Man 2 is a good movie on its own and serves to hype next year's Thor and 2012's The Avengers.