Hmmm... tough question.
In order to give the proper response, I think we need to look at the thresholds for different ratings. Assuming we're dealing with North American rating standards, as I understand it, there are three different measures that can bump a film up the ratings:
1. Sex and Nudity
2. Violence and Gore
3. Drug Use
Depending on the jurisdiction one is in, there are different standards for these so I'll just describe how the games I run would rate on each of the three axes:
Sex and Nudity: My games are as explicit as Jame Bond movies but a good deal less concerned with sex. There are occasional allusions to sex but no sex scenes involving the characters, except, perhaps the kind of implied sex from 1950s movies with the rapid fade to black followed by the crashing waves on the beach. Minus the crashing waves. That stated, my games are not quite G-rated because they do share the Bond phenomenon of double entendres and occasionally (every 5 years or so) have something shocking that pertains to sexuality such as the campaign I recently ran where the characters end up in a distopian future where there are periodic fashion crazes for body modifications, one of which was for manga-style hermaphrodites. So, I guess I'm PG here.
Violence and Gore: My games have masses of violence but because of my alientation from the D&D hit point mechanic (ie. it's too abstract to describe anything), the violence is not at all graphic. Aside from foes dying at the end, it's no bloodier than the parry-parry-kick violence of Xena. That is about to change, however, as I'll be switching back to good ol' Runequest combat rules in my next campaign, complete with hit locations and flying severed limbs. So, I'm expecting a bump from PG rated violence to PG-13.
Drug Use: Because I have pretty left-of-centre views on drug issues, drugs, paradoxically, don't play much of a role in my campaign. After all, they are just drugs. It's not like they, themselves, are intrinsically addictive or have power over people in any other ways. While some drugs are more addictogenic than others, ultimately, only a small portion of the people exposed to them will be addicted -- so I just can't write an evil drug plot. And if one can't do that, what is their narrative function beyond characters unwinding at the tavern after a long week of adventuring, something that rarely gets played-out. Also, D&D is full of spells and magic items that are the equivalent of different stimulant drugs so there is little need to replicate the short-term effects of stimulants when there's Bull's Strength, Cat's Grace, Owl's Wisdom (ritalin

), etc. In this area, then, I figure I'm at a G-rating.
I suppose I end up here with a game that is rated PG, that is about to move up to PG-13. I work that way because that level of ickiness is best for the social dynamic of the game not because I have young players or something.