There is little doubt that this is the movie that changed Hollywood franchises.
The original X-Men movie renewed interest in comic book films after the disastrous Batman and Robin. But Iron Man changed everything.
In retrospect, you can see the brilliance of many of the choices, but so much of what was done was a gamble.
I have always maintained that at least half of the success of the entire Marvel Cinematic Marvel Universe is due to Robert Downey Jr. Without him, I do not think you would have the MCU.
But it is important to remember what a risk he was to cast. The studios always want to cast younger actors so that as the years go on, the heroes don't age out too quickly. This can often have disastrous effects like Kate Boseworth as Lois Lane. Instead, director Jon Favreau wanted Downey Jr. who was already in his 40's. On top of that, it is important to remember that Downey Jr. had not been doing mostly supporting roles at this time or leads in smaller productions. And don't forget about his very public issues with drugs and prison.
In addition to this, Iron Man was not one of the A-List Marvel heroes. Director Favreau had only directed three movies up until this point, two of which had only barely made back its budget. The studio was also opening the movie only a week an a half before the hugely anticipated (though eventually derided) Indiana Jones and the Kingdom of the Crystal Skull.
Despite all of that, Iron Man rose to the top.
The movie does something that I always advise filmmakers with unlikeable leads to do: make your leads charming. Tony is not a likable person when the movie starts. He is a lothario who is thoughtless, greedy, and arrogant. If these were his only qualities, audiences would check out of the story immediately. But Downey Jr. makes you like him despite his flaws. The opening scene is so brilliant because it does two things: first it makes you attach to Tony because he is the cool/funny guy who is being cool and funny with us "normal" people. Second, he is immeadelty put into mortal danger so we feel for his safety right away.
The entire first act of Iron Man is fantastic. Downey Jr.'s chemistry with Yinsen (Shaun Toub) is the heart of the movie. Toub's understated and noble performance works to break through the snarky and sarcastic exterior of Tony. Not to enter into the realm of politics, but the movie came out towards the end of the George W. Bush administration when terrorism was a primary focus. By setting Tony's capture in the mountains of Afghanistan by radical Islamists, it struck a collective chord with a lot of people and gave us a superhero that felt like he was the enemy to our enemies. It was the movie equivalent of the cover of Captain America #1 where he punches Hitler.
The sequence of the escape is also fantastic, as we see our hero turn the tables on the villains and use his genius to overcome them and be the architect for his own escape along with Yinsen. After watching Tony suffer so much at the hands of these terrorists, it is a wonderfully cathartic release to see him exact justice.
But the goodbye to Yinsen always gets me. Again, Downey Jr. needs so much credit here. He can crank up the charm to 11, but he can also bring the proper emotion. Watch when he says to him, "Thank you for saving me." You can see how he means more than just saving his life. Yinsen saved his soul. It is a fantastic moment, not overdone or underdone. It is a moment of man who feels overwhelming gratitude and genuine emotion but doesn't really know how to do it anymore. His feelings are almost alien to him as he confesses his thanks. So when Yinsen says, "Don't wast your life," you can see how it propels the rest of the movie.
It is true that the second two acts do not match the level of the first act, but there is still much to admire. The development of the armor is fascinating to watch. This is not an easy feat because these sequences are essentially a man in a workshop building stuff. But Favreau makes each step of the journey fascinating to watch. And his first flight is still a real joy to watch, you feel Tony's exhilaration as he shouts with joy.
Underneath this is a story of redemption. Tony has been given a second chance to make his life count. The movie is about the slow awakening of his soul. The story wisely gives his character room to grow over the course of several films, but his first strides feel significant. It is wonderful to watch his first "metanoia," which is a Greek word for "complete change of heart." This happens thematically and symbolically with his Arc Light in his chest. His past sins are killing him, but his current heroism is saving him. But if he ever stops being a hero, he will slide back into his deadly sin.
And his first real fight in the new armor is great to watch. Again, Iron Man is not fighting a super-villain here, but Islamist (alebiet in this case fictional) terrorists. You get the sense of a hero touching, tangentially, the evils of the world.
The movie's biggest detriment is in its villain. Jeff Bridges' Obediah Stane is too simple and obviously evil. That isn't to say that Bridges doesn't do the best he has with that material. Gwyneth Paltrow as Pepper Potts has great chemistry with Downey Jr. Even Terrance Howard does a decent job as James Rhodes.
Also I think people forget how bold the ending is with Tony eschewing the necessity of a secret identity. While that is standard for most Marvel movies now, it was a brand new idea.
But above all of this, Iron Man is a fun movie. Never underestimate the power of fun entertainment. That doesn't undercut any of the drama involved, but most people finish Iron Man and they feel charmed and exhilarated. This movie permanently planted Robert Downey Jr. into their affections, and by association Iron Man and the entire MCU.
Nearly ten years later, Iron Man is still one of the best in the genre and deserves its place as the eight greatest superhero film of al time.