1: Don't interrupt other players.
Although roleplay truly shines the most when it involves complex interaction between multiple characters, several of which most often being players, one should always be mindful not to just barge into interactions which your character isn't already a part of.
I have often seen people suddenly appearing in the middle of an intense argument or epic battle and being all like 'HEY GUYS LOOK AT ME I AM HERE NOW AS WELL'. Don't do this without good reason.
2: An overpowered character isn't fun for anyone.
It's no fun to be part of the same roleplaying group that All-Powerful'. Most DMs will keep you from making a character which is too far over the top, but a good roleplayer should always try to create a balanced character with interesting strengths and weaknesses.
Roleplaying will be more fun for you as well as your fellow rolerplayers if you don't just lawnmow every challenge. The greatest fun in roleplaying isn't vanquishing endless amounts of foes without breaking a sweat. Sure, that can be fun too, but if you ask me there's much more enjoyment in getting into trouble and using your wicked cunning (and/or dice-rolling skills) to get out of it and helping together to overcome obstacles.
3: Think of your character as a real person.
Unless your character is god, or perhaps especially if your character is a god, it is important for a good roleplaying experience that he/she has a believable personality.
There's no need for every character to have some sort of crippling phobia, but if everyone's a generic grizzled veteran with no fears of anything and who has no problems dealing with any kind of situation then your roleplay probably won't be one for the story books.
4: Don't force trying to make your character in the center of attention in a roleplay, just don't do this.
5: Knowledge is also power. See rule #2.
You should always make an effort to think about what information is actually available to your character when deciding on what to do. If you DM doesn't limit your character in what knowledge is available to you for you then you should do so yourself, as a character that knows everything is just as annoying to play with as a character that kills everything.
Even if your character is a master farseer or fortune teller, for the love of the raves which watch my windowsill do NOT have his default response to everything be 'Just as I foresaw!' or something along those lines. Seriously, this is surprisingly common.
6: Mind your language.
Be mindful of how your character would realistically speak. A medieval knight won't say things the same way a FUTURE person from modern times would.
Obviously it's hard to be completely realistic here, but it's worth it to give it your best shot.
7: Know your OOCs and ICs.
Always make sure that it is abundantly clear what is being said or done by your character and what is being said by YOU. A good way to distinguish between the two is to preface them with 'OOC' (Out Of Character) and 'IC' (In Character) tags.
8: Divine Showdowns = Serious Business.
When two extremely powerful forces collide, it will not be something like "X Punches Y, Y dies. Everyone is happy."
There's going to be consequences, and chances are a lot of people won't like 'em.
9: Play a fair game.
Note that an inactive player is inactive and any actions taken against him/her is Unfair. Until he/she is active again, you cannot take any actions towards his/her Characters nor Kingdom.
10: Permission from the Leader
Joining a faction, doesn't mean you will have control of all faction unit, you must ask for a large army. But, if only a little army needed, you could take that.
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