It's Loki and his peeps.
And it's all the actors from the Marvel pre-Avengers movies. A lot of Whedon's genius comes from the group interaction, IMHO, and this is an ensemble cast. Except a lot of them are super egotistical, so I'm not sure how well that'll work. However, with Mark Ruffalo...*spaces out, smiling wistfully*
I think that Mark Ruffalo actually got involved because of the previous Bruce (Edward Norton) couldn't play well with others. But that's just Internet rumor, so who knows.
Really, the movie could totally suck and it would still be worth seeing. It has enough eye candy to keep me happy.