Washington Post:
MARVEL STUDIOS gave us fair warning.

Back in 2008, at the end of the first “Iron Man,” Samuel L. Jackson’s Nick Fury said to Robert Downey Jr.’s billionaire industrialist: “I am Iron Man? You think you’re the only superhero in the world? Mr. Stark, you’ve become part of a bigger universe. You just don’t know it yet. ... I’m here to talk to you about the Avenger Initiative.”

Those words launched a studio’s real initiative — Marvel’s bold plan to roll out four more films: an Iron Man sequel (which would debut Scarlett Johansson as Black Widow) and movies featuring the Incredible Hulk, Thor (which briefly introduced us to Hawkeye) and Captain America, all linked with the presence of S.H.I.E.L.D (as well as hidden scenes at the end of each film). Then, the biggest step: Assemble all the heroes together for a super-film. (Cinema, Marvel Studios, The Avengers)