I honestly have mixed feelings about Joss directing The Avengers. The pros are that he can do team dynamics very well and is a very capable director, even under limited circumstances (TV budgets, lower than normal budget for Serenity, no-budget Dr. Horrible). He's got a nice run of X-Men comics on his writing resume, so he's familiar with Marvel and has a seemingly good relationship with them.
The cons...after watching his work for years, I've noticed some annoying trends that creep up in his work repeatedly. A couple examples: He seems to hate happy relationships. No couple in Buffy or Angel stay together in the end and Serenity ends with a happy relationship beginning, but had the series continued, Simon and Kaylee would likely have not stayed together. He also plays the "kill a major character to shock the audience" card too much. As much as I hated it, I've accepted that killing Wash was the right move for Serenity. But it loses its potency after he had played that trick so many times before. Not only that, the climax of Dr. Horrible hinges on both of these Whedon tricks (death of Penny).
Yes, Zoe and Wash were an awesome married couple with interesting dynamics and plot lines. And then Wash was killed. Goodbye relationship.
That's how half the relationships went in Buffy and Angel. They were either your standard we can't be together sort of thing (Buffy and Angel) or these two are happy so one must die (Willow and Tara). At the end of both shows, no relationship was left intact. And that's fine a time or two, but I'm just saying that it's becoming easier for me to telegraph where Whedon is going with his writing because he's revisiting his tricks too often.
All that said, I feel like Joss is the kind of guy who should be creating his own thing rather than playing with other people's toys. But maybe this will be good for him and he can use The Avengers to do a Nolanesque move and bounce between a franchise (like Nolan is doing with Batman) and original Joss material (like Nolan is doing with The Prestige or Inception).