Good question!
Basically:- No overt porn and nothing illegal.
- Keep it about games
- Keep the stuff that pushes the limit here
For more detail, I like the description that ESRB Rating uses:
MATURE
Titles rated M (Mature) have content that may be suitable for persons ages 17 and older. Titles in this category may contain intense violence, blood and gore, sexual content and/or strong language.
If it seems rules need to be added, we'll look at them, but for now, our golden rule stands here as well as everywhere-- Don't be a Douche. And when someone else is a Douche, fight with the post, not the person.