I might be the only one, but does anyone feel like there's a chance of this absolutely failing? That isn't even a slight against Joss Whedon (the man is amazing), it's just I don't really see how something like "The Avengers" could actually work in a film medium. Part of the appeal of a comic-book is that story arcs can play out through multiple issues, lasting months, even years. Obviously, a film doesn't have that luxury. I'm just afraid that this movie will turn out to be like The Expendables: a film hyped on the fact that its a massive team-up film, yet only the A-listers (in this case, RDJ and Chris Evans) get most of the screen time, while everyone else gets what amounts to a guest appearance.
I think it absolutely can fail, but I anticipate it will be on par with the other Marvel films. Good, but not great unless it is absolutely your thing*. I think that with the possible exception of the first Iron Man, that's a fair description of all the films in the franchise. Solid.
I don't think the bloat will be a problem. Whedon's done his fair share of ensemble work (read: just about everything he's ever done). I think threat will be the problem. When it comes trailer time, I want to see images of these people that have all been borderline invincible in their films looking terrified and/or beaten up.
*Full disclosure: these films are absolutely my thing.