Iron Man is my favorite superhero film. For one, it has a real A-List star in Robert Downey Jr. (he was always one of my fave actors since I first saw him in Chances Are then with Air America and Chaplin) although he was on the comeback trail at this point following his lengthy bouts with substance abuse. As Iron Man director Jon Favreau said, he made Iron Man believable and added credibility to the film.
Two, it incorporated many elements of what made Marvel Comics the success it is today -- flawed real people in real life situations with a healthy dose of humor. It has a great script and plot and even better action sequences. The infusion of the US military especially in the Mid-East theater made it even more believable and timely. Director Jon Favreau who I first became a fan with when he starred and wrote Swingers is a fave. Iron Man is his best work.
Three, today's technology allowed such a film to be pulled off and seven years after its initial release, it still makes for great viewing.
Four, it opened what is the expanded cinematic universe and was the first Marvel film with its now trademark post-credits scenes.
And lastly, the music. Superhero film adaptations didn't use popular music. Iron Man made AC/DC viable again with its classic song, "Back in Black." And it also closed out the film with Black Sabbath's "Iron Man."
Plus, I love the cameos! Iron Man featured not only the co-father of the Marvel Universe in Stan Lee but Rage Against the Machine guitarist Tom Morello (see pics below) was in the film as "Insurgent #5.'