1: Interrupting others.
It is mainly a bad idea to interrupt other players while they are roleplaying. If you just crash in and roar "The orcs are coming!", make sure the orcs ARE coming. If you dont have a proper reason for interrupting, it is a SERIOUSLY bad idea. Sometimes people are having some real nice plot developments going on, and then the only proper thing to do in most cases is DONT INTERRUPT. Ofcourse interrupting can always be justified with a REALLY good reason.
2: Imbaness.
If you are the Evil Arch Demon of Ubarland, make sure you have a good reason. If your charecter is ultra imba in combat, perhaps make him abit stupid. Things like extreme arrogance is just... Frustrating, when it is combined with imba power. So please, if you dont want to be shunned and find yourself a loner that nobody wants to RP with, think about how your charecter is put together.
3: Personality.
Unless your charecter is god, he will not have all values as Bravery, Strength, Wisdom, Intellect, Magic, etc at a level about "Sir Worldchampion II". Make sure your charecter has flaws. A flawless charecter is just... bawring?
4: Centering.
Do NOT try to center the rp on your charecter. The RP will AUTOMATICLY center on one of the charecters, and do NOT try to change that. Appearing everywhere something nice is happening is just... bad (im still having problems with that one myself) and i do NOT reccomend it.
5: I-Know-Everything Guy = Bad.
Even if your charecter is a master farseer or fortune teller, do NOT make your charecter say "Ah, just as i expected" to everything! It is LAHME!
6: 1337 Sp43k.
Making your charecter say things like "Wtf?!" makes him/her very hard to take seriously. Even phrases like "2 guards still remain" is abit lame. So i reccomend that you think before you write, and do not use shortenings or numbers in charecter speech.
7: OOC.
OOC and IC is a great system for telling if your charecter is saying something, or YOU are saying something. Basicly it means Out Of Charecter and In Charecter. If you have your own little system for such things, do not use it unless everyone else in your rp thread use it too. In fact the "owner" of the rp thread should decide what should be used and what shouldnt.
8: God-Emoting and Power-emoting.
Guys that power emote or god emote are those who write things such as "{insert random name here} kills/decacipates/impales {random guy here}".These will not only make people angry because their rp character died, but also a person using this kind of emotes will undermine his rp reputation.
If you and another RP'ers characters are interacting in any way, (which happens very frequently) don't assume control of the other's character for ANY REASON. If your ninja-elf tosses a dagger at his warrior, you don't know what he wants to do, so don't add "and Warrior ducks to the side and charges Ninja-Elf." You don't know what the other person wants to do; he could snach the dart out of the air or even want to take the blow. Controlling other's characters can, and will, lead to harsh words and someone getting upset, so just avoid it.
Credits to OMnaxos and Crazy Cow.
9: Divine Showdowns = Serious Buisness.
If two extremely powerfull forces fight, it will not be something like "The God of war hauls his spear at the Demon, and the demon dies. Everyone is happy.", it will be something like "The God of war hauls is spear at the demon, and the demon explodes in a fiery vortex of destruction, creating a void rift". So remember: dont just type something like: "the two titans battled, and one of them died." since that is lahme.
Credits to SoLMaster.