I've been using the fan rating system on my archive site (FRC, FRT, FRM, FRAO) because that's what giles_watchers was using when I first began helping to compile that newsletter. It seems to be falling out of favor in fandom, however, and it's never really satisfied me. It requires explanation in a way that simple English words don't.
American movie ratings mostly satisfy that no-explanation requirement as well, because of familiarity. Four categories: general audiences, parental guidance suggested, restricted, adult. That feels like one too many categories to me. Fandom probably doesn't actually care about the teen or child audience distinction. Fandom ratings are like fandom warnings: labels that help us choose fic to read that suits our mood. Do we ever really worry about PG vs G?
What problem are we attempting to solve when we rate our fic? It's not the same problem movie ratings are intended to solve, of preventing young viewers from seeing content that society has decided they're not ready for. Fan readers are making choices for themselves. Fic ratings are trying to solve the problem of matching readers to content they want. Viewed this way, "R" is just another content label the way "fluff" or "hurt/comfort" would be, though a more vague one.
I'll bet that you as my readers usually don't care about anything other than prawny-intent vs no-prawny-intent, but let's find out. Do you ever pick something G-rated to read because of the rating? How do ratings affect your reading choices? Or are you more likely to look for other content, like "I want an angsty first-time story today"? (I realize that pairing is the single most important driver here, but let's assume that you're selecting among stories that have a pairing you like.)
I'm thinking of going with just general/mature/adult, in order of increasing difficulty of material. (Bah, my need for parallelism is not satisfied by that list.) Or I could be lazy and do nothing at all.
Listen: "Vapour Trail", Ride, Nowhere. Live version for the Rickenbacker pr0n.