For a more in-depth guide, please turn your eyes at the So You Want to Roleplay?
topic. No ruining other people's fun
Roleplaying is supposed to be fun, so please try not to ruin it for others. This includes interrupting an emotional or otherwise important moment or just plain ruining the mood of a game. If you don't like the mood a specific RP has at the time, feel free to create a RP of your own. Try to avoid godmodding, unless you have the permission of the players of all affected characters
Always ask for permission first before controlling other people's characters. No one likes to come into a RP only to find that their character crippled or even killed without them even knowing. No one is invincible!
No one should be invincible, unless everyone has agreed to it! If your character is in combat, he/she WILL be wounded in the process. Likewise not all of his hits will land.
If there is a general rule that should be mentioned here but isn't, let us know about it.