The plot is reasonably faithful to the comic books, and that was a big plus. Very very well-paced, and the plot transitions were extremely good and easy to follow.
The characters were superbly played. Who would have thought that Robert Downey, Jr would be able to play a superhero? Amazing. And finally! An actress who portrays a character in a very sexy but not the least bit slutty role. Casting Gwyneth Paltrow was definitely a very good decision. She played her role much better than how Kirsten Dunst played Mary Jane Watson in the Spiderman trilogy. However, my opinion does not necessarily reflect anyone else's. ;)
I love the technology and the effects. The 'assembly-line' method of Tony Sparks putting on his armor was brilliant! The way that his armor worked was also convincingly explained and carried out, leading (I believe) many viewers not to think 'Wow, that is a load of bollocks'. The special effects team were incredible.
Fight scenes between Iron Man and Iron Monger was alright. Nothing special.
I must say, the ending was brilliant. :) You have to watch it to see it.
You will be caught off-guard and wanting more, especially if you weren't expecting it, and I can say, no one did. :)
All in all a good movie, and definitely made the summer of 2008 very enjoyable.