Usually, I go into comic book movies with a big tub of popcorn, some juice, and the knowledge that I'm about to get some fun, plot-light action movie with lots of superpowers and/or explosions. And that's fine by me. I know what I'm in for, and to me, the Marvel movies have all delivered a fun, worthwhile experience to enjoy.
Iron Man went further, which really surprised me. It's not that the movie wasn't fun (it was), nor that it wasn't witty and light (it was both), nor that things didn't explode (boy, did they), but that the character of Tony Stark, and the actual plot of the tale was, well, endearing. It was a good movie, not just a good comic book movie.
So, if you've enjoyed the Marvel movies of the past, you're in for a treat. Not since the first Spiderman have I been this surprised at the "extra" put forth in the film.