This parents' guide for whether or not to let their kids see Sin City is pretty funny; they detail each scene of violence, which does a pretty good job of summarizing the movie:
http://www.kids-in-mind.com/s/sincity.htm Also: "DISCUSSION TOPICS - Murder, revenge, love, torture, cannibalism, law and order, corruption, psychopaths, being framed for murder, child molestation, rape, serial killing, torment, pity, racism, humiliation. MESSAGE - Love can grow even out of the darkest circumstances."
That's not exactly the message I got, but whatever.
All in all, though, it's a good resource and their scheme (re-calibrated for general use) should be used instead of the current ratings. Their dataset would also be nice to run a regression on, to use their more detailed descriptions (including interactions) as predictors for receiving an R-or-worse rating. It's obvious that sex will "matter" more than violence, but we could also model time trends, and look for interactions (does a religious theme make profanity better or worse, &c.).