I saw Iron Man twice in the theater when it was released. As far as comic book origin movies go, it's without question one of the best in the business. Great casting, solid story, good pacing and a lot of heart where it was needed. To this day I'm sure Robert Downey Jr. is still thankful for what that movie has done to his now prolific career in Hollywood. At least I hope he is. It's not everyday that you get to have your career revived in such a way that now when people see you or hear your name, you are synonymous with a comic book hero for the rest of your life. He is and forever will be known as Tony Stark/Iron Man, no matter what happens to the Marvel Cinematic Universe or Marvel Studios in general. That's pretty awesome.
The thing about Iron Man that really makes it work is that it's a well told story visually and in terms of details of the story and characters. The themes and ideas are pretty well laid out. There's the arc of redemption and accountability for Tony Stark when he discovers that his weapons made by his father's company are killing countless innocent lives across the planet, which leads him ironically to create an even more powerful "weapon" in his zeal to atone for his family's sins. It's a story that people can relate to even though he's a billionaire jackass because he's a heavily flawed person and every one of his flaws are on display for the audience to see up close and personal the entire time.
The biggest thing that helps this is the cave sequence. From the moment Stark is captured by The Ten Rings and forced to do their bidding and his world is turned upside down, we see a wholly hedonistic and self-absorbed man forced to face the consequences of his actions and inactions whether he likes it or not, and he doesn't like what he sees. In fact he hates it so much that it drives him to change things on his own later. We see his motivation change from being extrinsic and superficial in the beginning to being intrinsic and self-sacrificing later. The way this is accomplished in the movie is well done too, with it being mostly visual and auditory instead of needing a lot of exposition. Iron Man isn't a movie that explains a great deal to you with broad sweeping speeches or a ton of dialogue. No, you see very clearly on screen what is happening, why it is happening and what effect it has on everyone and everything else.
From the beginning, Iron Man made it clear that the Marvel Cinematic Universe was going to be an action movie franchise steeped in comic book lore and adaptation. It's not going to be a franchise that is replete with easter eggs and a ton of references to the source material. Those things are there, but the aesthetic itself is pure blockbuster action movie from top to bottom. In Iron Man's case though, it's an action movie with a solid story that isn't glossed over and pays attention to the performances of the actors and the development of its characters. It's not a movie that flies by too fast at all in 126 minutes. It takes its time to setup and flesh out the right elements for the story and give the audience enough decompression in between action beats to get the big picture.
Iron Man is also one of the more serious and straightforward movies in the MCU, which is a trademark of Phase One in general. At the time, the MCU had humor in it, especially where Stark's character was involved, but it was timely humor that served to support the situation and the characters instead of being a driving force of the story itself. You didn't like Iron Man because it was hilarious, you liked it because it was entertaining and told a good story that was funny at times. That's the model that just about the entire Phase One of the MCU followed and it was also the embodiment of comic book movies at the time in terms of how they were starting to be built and appraised by the audience.
The Stan Lee cameo with him being "mistaken" for Hugh Hefner is solid. The visual effects and CGI for the time were certainly top notch and still hold up today. Ramin Djawadi's score is standard issue action movie template but it works for what the movie is, without question. As one of the few MCU movies to be shot on film, Iron Man is still one of the franchise's top film without a doubt and given its $318.4 million domestic gross and $585.1 million worldwide box office tally, there's no question why it was a great starting point for the MCU as a franchise.