The Avengers leverages the previous films for its primary characterization beats, and starts here by continuing some of those arcs - albeit in simple swipes. Nevertheless, maybe because of that, it took a little bit for Avengers to find its groove for me, rushing on the screen with a frenzy of plot establishment with characters from all over the Marvel universe, and I found it difficult trying to establish my bearings with it all (which, yes, can lead to a bit of dull movie viewing). But as each character was wrangled in by SHIELD, the rhythm and energy became more consistent - and Whedon's little touches became more evident. And with the aforementioned updates on character continuation, I found the interaction between all the superheroes, etc., a lot of fun, if not too terribly revealing.
Still, the fact that Whedon is able to balance each of the superheroes - both in their motivations and in their style of action - in a massive franchise-peaking picture like this with a coherent plot and ample stakes, is pretty impressive. There isn't a ton of room for the usual Whedon stuff; it's just about delivering something that makes sense. Everybody gets a fairly equal share of presence and conflict, and the forces of evil seem so ratcheted up, with the volume almost up to 11, that the Avengers' collaboration feels well-placed for the proceedings. And while this film wasn't really of strong emotional resonance, the setpieces and the interactive action were often outstanding, combined with the humor and the strong character touches, it netted out to a pretty stirring effect. Had a lot of great, breezy momentum, and finished just as you would expect most fun blockbusters.
It's low-calorie, but I don't know that it's such a bad thing for this kind of grandiose superhero film. The sense of consequence didn't feel right given the scope, though, and there are a few plot advancement moments that don't work. That said, I would so watch that again.