In many ways, the true triumph of the Avengers is not on the screen, but rather what happened behind the scenes, the deliberate planning and organisation of four large films so that characters could be introduced, ideas seeded, and then brought together in one film. Perhaps, even, the planning for the films could be suggested to have taken place a lot earlier, in the Mark Millar and Bryan Hitch Ultimates, where Nick Fury, suddenly black, is so clearly modeled after Samuel L. Jackson. That would place the planning of the film--the expansion of the Marvel franchise--at a decade, which may or may not be reasonable. Perhaps more telling in a reason not to think this is that Millar has a long history of modeling characters after actors and singers, partly, if I remember right, with the aim to sell them more easily to studios.
Nevertheless, the Avengers does represent a planning success, where an entire franchise is developed in not just one film, but four proceeding ones, which results in a film that can have sixty percent of its time having the protagonists fight each other, while the remaining forty percent of the time is them fight their common enemy.
Given then that the true success of the film is that of franchise, what then, can be said of it as a work of art?
Truthfully, not much.
It's not a bad film. There's nothing wrong with it, really. The plot is silly, but no more so than a thousand other films, and for a film that threatens an alien invasion on Earth, the invasion is pretty small, the aliens largely undefined. The dialogue is decent enough, the characterisation suitable, and the action moves along at a reasonable rate. Like I said, there's nothing wrong with it. It could have been written and directed by any number of Hollywood hacks--and that perhaps is the cause for disappointment, because you hope, somewhere, that director and writer Joss Whedon, who has been responsible for some fairly creative episodes of TV (including my favourite, the musical Buffy), could have bought some of that to the screen.
But in the end, it doesn't truly matter that he doesn't do this. It's a decent film and it firmly establishes Marvel not just as a franchise for films, but as a film franchise that has a wider spread than Batman or Superman.
In other words, learn to love the company.