The concept of this movie is what's going to lock me into seeing it. Outside of the Hulk, who for reasons everyone already knows has been recast yet again. I find it really fascinating that we're getting characters from what is essentially four different movies combined into one film. I'm pretty sure this is the first time this has happened, correct me if I'm wrong. That fact alone is making this a must see for me.
On top of that I'm really intrigued with what Whedon is going to bring to the table, as he hasn't had a crack at a feature film since Serenity a few years back, I hope this is the film that will allow him to break into Hollywood.