Iron Man set so many precedents for the MCU. The humor, the action, and the villain have all laid the groundwork for future entries. RDJ is great as Tony Stark, you see the groundwork of the character that will eventually form his character in the future events of the universe. The loss of his parents likely played a role in his desire to protect people. Even his desire to sell weapons was born less out of greed and more out of a desire to protect the soldiers. It's that same desire that leads to Ultron and signing the Sokovia Accords. It's also why he's so over protective of Peter Parker in Homecoming. You also see the anger within him when he sees the terrorists hurting people with his weapons. He doesn't think, he throws on the suit and goes into action. You see him display similar emotions with Loki in The Avengers, and when he turns on Bucky and Cap in Civil War.
His relationship with Pepper has great chemistry, but it's never really anything more than flirting or a show in interest within this movie. The whole boss-assistant relationship does ring a little awkward in the modern climate. It's probably why the character takes a larger role in future movies. They're still the second best couple in the MCU.
This movie is also a great snap shot of 2000s America. It wallows in an America that's gung-ho about carrying the biggest gun to combat threats. It made for an interesting watch for a movie that came out towards the end of the Bush era.
I still really enjoy this movie, but the future entries in the universe build off its successes and make them better.