Five years ago, Joss Whedon's career wasn't going so well. The man behind the TV shows Buffy the Vampire Slayer and Angel had recently written and directed Serenity, a big-screen version of his short-lived series Firefly. It was his feature directing debut, and it was a commercial disappointment. Not long after, Warner …

Tomorrow, Marvel's The Avengers will open in certain parts of the globe—not the U.S., which won't see the film until May 4—and Marvel's four-year odyssey of laying the groundwork for this action blockbuster will finally be over. It's been an impressive run commercially, if not always critically. (Captain America: The…