If Captain America: The First Avengers had been the first film in the MCU instead of Iron Man, how would it have affected the MCU?
To the max.
As much as I love Captain America and his first film, there's a difference between opening with a bang and opening without anyone taking the time to notice.
Iron Man started a cultural phenomenon.
That may sound exaggerated, but look around you.
Superheroes are everywhere.
Because Tony Stark revolutionized them. What RDJ and Marvel created those ten years ago was a gamble. They were playing an intricate game that no one expected them to win.
People went out to see Iron Man because it was a different type of superhero movie.
The key to the success of that film was that it was everything superhero films that came before it weren't.
Marvel capitalized on this. They came onto the scene as the underdog with an unorthodox hero. There were no expectations.
Captain America is a clean cut hero. You can guess what happens next. He doesn't have the charm of Tony Stark or of RDJ.
If The First Avenger had been the first Marvel film of the MCU, there would be no MCU.