Why? Whedon is a writer-director-creator with a massive imagination and loyal fanbase. But he has fared best away from group-think and commercial considerations. He's not a studio factory player (see Alien: Resurrection or Titan A.E). He doesn't think in conventional commercial mass-audience terms. (Nothing came of his stabs at Atlantis or Wonder Woman.) He's best at creating strong relatable characters in such TV series as Buffy the Vampire Slayer, Angel and Firefly, which flopped in movie form as Serenity, his one feature directing effort. Whedon's most commercial screen credit: he contributed to the script of the original Toy Story. I'm curious to see how his screenplay for The Cabin the Woods turns out for MGM.
While fan sites are applauding, The Playlist sounds a note of caution. HitFix's Gregory Ellwood (@hitfixgregory) tweets: Whedon has "written for Marvel Comics and knows the universe, very very well. Not as surprising as you might think." And Harry Knowles (@headgeek666) reminds: "How can any of use pretend to know what a fully financed Joss Whedon film is? I have never seen one."
UPDATE: Now Pajiba reports that Whedon will also do a rewrite on Captain America. Makes sense. With Marvel, it's all about keeping the universe consistent across all its media.