The hate for Serenity in fanboy land is new to me. I thought most who saw the movie, especially in the geek community, thoroughly enjoyed its mature, clever and sharp writing and impressive ensemble.
I say give him a chance. I again stress that he was picked because Marvel is looking for someone to do for Avengers what Favreau did for Iron Man and Abrams for Star Trek. When those names were signed fanboys whined as well. This guy knows his way around a camera and more importantly a strong cast. If the trailer looks cool and zippy (which I don't doubt) these fans whining will be drooling in hope when San Diego 2011 comes around. Remember, Whedon is a guy who loves pleassing fans and I don't have much doubt about this.
I'm honestly more surprised people are more excited about Joe Johnston who hasn't made a particularly good movie in a LONG TIME or Louis Letterer who still hasn't made one and their future work with Marvel movies.
And if someone is going to hold Alien 4 against him, can we not counter that in the same period he wrote THE screenplay for Toy Story? While a kids film (like Favreau's previous work) it is one of genuine heart and merit that remains arguably Pixar's best project to date as well.