Act One: In which a Threat emerges and some characters are introduced
Act Two (First Half): In which we witness a series of decidedly awkward encounters between characters
Act Two (Second Half): In which some stuff happens, and we observe the aftermath of that stuff.
Act Three: In which some further stuff occurs.
Here's my review of the movie:
The Avengers (2012) ***** Holy. Shit. Pacing issues don't detract from splendid characterisations and great action scenes. The bar is raised.
However, when you're watching it, notice what the antagonist does and doesn't do during the first half of Act Two. Ask yourself what their goals are, and whether they achieve them. Ask yourself: what is the immediate threat that the Avengers are dealing with? That's certainly what I'll be doing during my second and third viewing of the film - and it's the answers to those questions that created the intermittent bursts of slow pacing for me.
... Also: it's one of the funniest blockbusters I've seen in years.