1. No illegal content. 2. No malware or malicious mods. 3. Warn users if the mod may be destructive to their saves. 4. No sexually explicit content. Again, keep it PG. 5. State versions of the game your mod is compatible with. 6. State any dependencies your mod has (if any) and the authors of such mods (if the dependencies are mods). 7. Don't steal the works of others. Expanding upon someone else's mod by creating your own dependent mod is OK, but don't distribute other people's work without their permission.
If you are unsure that your mod meets these requirements, feel free to contact one of the Moderators and ask!