I think The Avengers is maybe the first true comic book movie. Not because previous movies based on comics weren't good enough, or faithful enough to the source material. But because seeing it feels like reading a comic.
One of the things that makes comics amazing is the tremendous range of possibilities available to creators. If you can think it up, someone can draw it. If someone can draw it, you can put it in your comic. Absurdly epic shit can go down, tons of it, all of it. You can introduce some fantastic location or prop or character and just toss it away or blow it up in the next panel, because ink is cheap. And you can do that over and over again, as much as the story calls for it. It's not just that anything can happen; it's that everything can happen. Provided you have a good artist, the only limits are the constraints of the format and the constraints of audience attention. (And hopefully those of good taste.)
Historically, movies are different. Resources are a far more limiting factor -- available technology, effects budgets, etc. Even serious blockbuster movies that are heavily built around action sequences and effects are usually pretty narrow in scope. You get a disaster movie, or a shoot 'em up, or a classic superhero movie where superpowered people throw each other into buildings. You get one of these things.
But The Avengers moves seamlessly, continuously from sci-fi action to disaster movie to military action to epic fantasy and back again, all while also doling out a tightly calibrated mix of comedy and drama. One minute you're in the middle of Star Wars; the next, you're in the middle of Lord of the Rings; the next, you're in the middle of Cloverfield. It's multiple overlapping epics epicing everywhere, all the time, epically.
That should render the movie totally inconsistent/incoherent, but it doesn't -- because the chaos of it provides its own consistency and because The Avengers is built around the understanding that character experience, not genre tropes, is what the audience is really tracking. And because people are always willing to suspend an incredible tonnage of disbelief as long you have the balls to ask them to and the chops to make sure they're having fun while they do it.
The result is not just a good movie with comic book characters in it. It's a movie that "reads" like a superhero comic. Which is really a sort of remarkable achievement, all things considered. I can't imagine anyone else but Whedon pulling it off.
This is not to say that it's a really important or powerful story. Honestly, it's not. It's maybe the most ideas-light thing in the entire Whedonverse. BUT IT IS JUST SO MUCH DAMN FUN.
Oh, but I still think Cobie Smulders is a terrible Maria Hill. Please, please recast that role.