With the disclaimer that I have never read comic books and have no idea how faithful to the stories the movies are...
I love Robert Downey Jr. as Iron Man. The character he creates is fantastic to me.
He is excellent as Iron Man. IIRC, he is under contract for just one more Iron Man role, which will be Avengers 3.
I think this may be the reasons in Age of Ultron, he is kinda leaving the picture with War Machine resuming in his place, IDK.
However, Tony Stark did suffer quite a bit of revision, this character is portrayed suffering darker chapters and personal demons in the comics. Even a serious bout of alcoholism.
Otherwise, Robert Downey Jr really cuts it up onscreen with the other heroes and his villains. This is what Deadpool and Spidey are supposed do (cut it up with wisecracks), but .... never had, sadly.
Likewise, Bruce Banner became the Hulk by pure accident after saving Rick Jones in the comic, he wasn't researching any super soldier serums like it is stated in the Avengers movies.
Likewise, Ultron isn't made of ultralight indestructible vibranium (sic) but indestructible adamantium (the same substance as Wolverine's claws and skeleton).
But these revisions are quite forgivable. Because the movies rock!
But don't get me started on the Mandarin though!