Via M. LeBlanc, the Bechdel Rule, from Alison Bechdel, the creator of Fun Home.
The rule is that movies should have 1) at least two women, 2) who talk to each other, 3) about something other than a man.
I'd never heard this articulated in this way before, but LeBlanc is right-- it gets at something important, and remarkably few mainstream movies satisfy it. I can't offhand think of any recent genre film besides "Stardust" that qualifies-- not even any of the X-Men movies, even though they had a number of women and weren't romance-focused. (Most conversations in those movies were with Wolverine, Magneto, or to a lesser extent Xavier, weren't they?) I guess the Next Generation Star Trek movies probably did, since Beverly and Deanna had an established friendship, but I can't think of the scenes offhand. If little girls count, then "The Golden Compass" and the Narnia films qualify.
I'm not going to use the Bechdel rule as a way to judge whether to go to a movie, but I do find it (and LeBlanc's discussion of it) immediately helpful as a way to think about what's often missing from movies.