Overall, the standard is PG-13. That is because this is a business website, a business that markets it's products to 13+.
That being said, we do allow games that are a bit more mature to be posted here. However, there must be a clear disclaimer in the headlines. As Philosophus Vagus said, it's rare that we have to deny projects based on violence, since really, it's hard to get too graphic with RM. Not impossible, I can think of one image in Game Map & Screenshots I had to hide, so it can be done, however, that's in a year and a half, the only thing too violent I've seen.
Language needs a disclaimer, and we would ask that it not be presented in screenshots of dialogue or trailers that are posted. Some curse words are okay. A screenshot of dialog where the "F word" is said 10 times, not so much. The same goes with sexual content. Girls in small bikinis is one thing. Girls in almost-non-existent bikinis in suggestive poses, not so much. And we have denied games that, while maybe not presented in the topic, were known to contain some pretty heavy sexual content.
Mostly just use common sense and if you're really in doubt, message a mod with questions.