I love all the hostility from the fanboy community. I"m not sure if they are angry that he only has done one movie or that much of his work involves women who aren't damsels in distress, but there is some white hot hatred for the guy. I'm surprised.
The man more than anything knows how to juggle an ensemble and make awesome, really endearing characters and relations. Serenity, when viewed in context with the brilliant series Firefly, was the best sci-fi film of the last decade. He makes viewers love his characters, even if there are nine of them. Each is individually developed, distinct and cool.
While I was never a huge fan of the Buffy series, he had that same sensibility. He has directed some solid action scenes before and I have confidence he can up the ante with a $200 million budget. The complaints that he can't seem to be from people who don't know his work or think because he hasn't made an action movie he can't do it. Because we all know how well Louis Letterer and Brett Ratner, directors with action experience, were with Marvel movies.
Whedon's Youtube movie/musical, Dr. Horrible's Singalong Blog, was better than half of the comic book movies made to date. I think he WILL BE FINE.
And for those who still whine about experience, I bet they were pissed off when JJ Abrhams, the guy who created the "girl power" show Alias and drama/mystery Lost got the job to do
Star Trek. He ended up making the best Trek movie ever, i.e. one that people actually wanted and enjoyed watchings. I'm also sure when "the guy who made Elf and Zathura" was given the job to make Iron Man, they wondered how "this small-time supporting actor and kids' movie director can handle Tony Stark? Where's Michael Bay?"
If you want Clash of the Titans, then I understand the problem. If you want something closer to Serenity or the last Star Trek movie, then Whedon is a great choice.