Assemble ensemble

Rating: 9/10 (24 April 2012)

The critics so far are basically right about what makes it great. Virtually all of the reviewers have said it has excellent dialogue and great action set pieces.
No one who is a fan of fantasy action movies could seriously deny how good this is as an action spectacle: the special effects are very good, and put to good use by Whedon and his team. The tracking shot through the midst of the final battle is amazing. The finale is a genuine classic battle scene, and is as good as anything that Steven Spielberg would have put together in the same circumstances.
The movie is funny enough that IMDb should add 'comedy' to its genre list. And all the other things the critics are repeating (so far, anyway) are true: mostly good acting, Ruffalo is especially good as Banner, character interactions are done very well, everyone gets enough to do that it feels like an ensemble movie, all of the title characters from the prior films are portrayed as well, or better than, in their own films blah blah blah...
So all the good stuff we've been hearing/reading is basically true. What about the bad? There's less consensus around the bad, but probably general agreement that the plot as such isn't all that. It's basically the story as told on the cover of The Avengers comic #1: a team of powerful, but not unstoppable, individuals gather together to fight a foe no hero could stop alone. As Time Out's reviewer noted, the plot is basically just a coat-hanger for Whedon to build his character arcs and action sequences. But The Avengers was always likely to be limited by the need to be an origin story of sorts, and so I wasn't really expecting a lot more from the plot on this occasion; and the storytelling itself is very good, the way the character arcs were moved forward and entwined, was done very well. The alien army was an adequate, but not very innovative way to get Loki reinforcements and get a big battle for the heroes at the climax.
The beginning was a bit clunky (several reviewers have used exactly that word) and I don't think it was a very helpful way to get audiences who hadn't seen the build-up movies into the film.
There are potentially interesting themes of control, power and freedom (as Patrick Samuel touched on in his review at Static Mass Emporium), but these are not given sufficient fleshing out, leaving the movie in the 'summer blockbuster' category, albeit a much, much better than average example of that kind.
Oh, and I'll say it again: The official title is stupid.
But overall, I'd give it a 9/10. (Probably a 7 to 8 for those who haven't read the comics or followed the movie franchise closely.)