Thor: The Dark World: Hemsworth, Portman, Hiddleston, Hopkins, Skarsgård, Elba, Christopher Eccleston, Adewale Akinnuoye-Agbaje, Dennings, Stevenson, Zachary Levi, Tadanobu Asano, Jaimie Alexander and Russo.
Captain America: The Winter Soldier: Evans, Johansson, Stan, Anthony Mackie, Smulders, Frank Grillo, Emily VanCamp, Atwell, Robert Redford and Jackson.
Money, Dear Boy: Paul Bettany has stated that he sees his role as JARVIS as simply a source of easy money and has never even seen the films.
The Other Darrin: Mark Ruffalo is this to Edward Norton as Bruce Banner in the Avengers movie.
And while Eric Bana's Hulk isn't part of MCU canon, it took place 5 years before Norton's film. Norton's Banner had been on the run for five years. (You might call it "canon-fodder.")
And before that, Don Cheadle replaced Terrence Howard as James Rhodes.
Howard Stark has been played by a different actor in each film he's appeared in (Iron Man, Iron Man 2, and Captain America). Somewhat justified on that last one, as Howard Stark was supposed to be about 20 years younger than he was in the Iron Man movies.
Zachary Levi replaced Joshua Dallas as Fandral thanks to his commitments to Once Upon a Time which is a reversal of The Other Marty, as Zachary Levi was originally cast as Fandral in Thor but couldn't because of Chuck related scheduling conflicts.
Sony were keeping their options open to cash in on a Spider-Man crossover, but their new reboot has proved enough of a success that they've decided not to cut the pie.
And speaking of Spidey, the Oscorp building was originally going to be in The Avengers, but the film was too far in post production to include into the movie.
A Runaways movie got as far as casting calls before they realized that it's just too complicated a mythology to dump into the franchise all at once (in the comics the plot ropes in time-travellers, mad scientists, aliens, crimelords, witches, mutants and pan-dimensional god-monsters, and that's just the origin), particularly before they'd even got the Avengers fully established. Of course now that they're brave enough to make something as obscure and bonkers as Guardians of the Galaxy it's safe to put it back on the "coming soon, maybe" pile.
The Wasp was initially going to be part of the team in The Avengers, and was even included in earlier drafts of the script. Joss Whedon is on record saying that he really enjoyed writing her, but ultimately had to cut her out of the script due to there being too many characters.
Of course, they recently reannounced the project for Runaways, and Wasp is all but confirmed for Ant-Man.
Then more comments came out that makes you wonder what comes first: Wonder Woman or Runaways, in film?
Emily Blunt was Marvel's first choice to play Black Widow, but she had to drop out because the filming conflicted with Gulliver's Travels which she was contractually obligated to.