As the title says, these are some serious rules. I'm just copying and pasting these things, and they can be found in the Roll-playing for Dummies thread in the RPG forum. These are just really great guidelines, some of which I have covered, but whoever wrote these said it all really great, probably better than I did. So here they are.
First off, let's start with the basics: vocabulary.
OT: Off-Topic. This is when you say something out of character or not directly relating to the RP.
GM: Game Master. This is the person who runs and supervises the game, throws challenges at the players, sets up the encounters, monitors the gameplay, and keeps everything running smoothly. In essence, the god of whichever game they're running. Be nice to them, as they hold the power of life and death over every character in the game. GMs generally do not actually play in the games they run.
PC: Player Character. This is pretty self-explanatory - a player character is a player's character.
NPC: Non-player Character. This is a character not controlled by any of the characters. Run by the GM, these characters (just like yours) can have a variety of attitudes, secrets, personalities, etc. Essentially, an NPC is any character not run by one of the players, and can range from a friendly ally to an apathetic townsperson to an outright hostile enemy.
Godmoding: Trying to be the GM. This will be explained in more detail later.
Give your GM something to work with. Just saying "Yeah, my guy kicks butt!" is not good enough. Why are they so kick-butt? What have they sacrificed to gain their combat skills? Do they have a personality? What about history? Do they even have one? Is there a reason they wanted to acquire these skills? Is there some sort of secret in their past for the other characters to wonder about? (i.e. "nobody knows how or where he first showed up, or where he acquired his temper...all that anyone knows is that the first man who made him angry was found the next morning pinned to a doorway with a dagger through his throat.") Give it some depth. A sentence or two isn't nearly enough to tell us who your character is. Remember, your character is an extension of yourself, and nobody is two-dimensional. Give us some detail...your character is a person in their own right, so treat them like it.
Keep it realistic. Weapons and armor are heavy stuff - think about it realistically. I know that it's very tempting to make your character a powerhouse with several dozen weapons, but think about it reasonably. If your character were real, would they really want to weigh themselves down with a greatsword, two longswords, three daggers the size of shortswords, a couple of two-foot long kamas, a battleaxe, and three dozen throwing daggers? It doesn't make sense - a realistic character might have one primary weapon and two secondaries, or one primary and one ranged...at most. Keep it realistic. Before you deck out your character, imagine them in real life and think about how well they would function, how they'd fight, how they'd travel...half the trick of believable RPing is making your character real - they're not words on a computer screen or numbers on a sheet of paper, they're a living, breathing person as real as you or me. You don't have to make it totally realistic - after all, half the fun of RPing is pretending - but keep it real enough to be within the realm of plausibility.
This should never happen in an RP thread, that's what separate threads are for. It's a matter of etiquette. At the very least, before anyone says anything off-topic, label it as such. An "off-topic" in italics at the top would work, as would putting it in parentheses and sticking an "OT:" in front of it. But keep the story going...too much off-topic ruins it for everyone.
All too often, people like to assume other people's actions - even NPCs are controlled by the GM, and thus count as characters. As such, instead of "I slice into the guy's arm with my katana," a player should say "I swing my katana towards my opponent, aiming to hit their arm", and let the GM or the opposing character take it from there. This is a huge problem with most new RPers, and although easily solved, it tends to be the most common mistake made in the roleplaying world, even among those with extensive experience and skill in roleplaying.
Also a huge problem. A good RP should read like a novel. If the posts aren't in-depth, deeply thought-out, and well and carefully written, the RP won't be any fun for anyone who's played a quality RP before (as I have on more than a few occasions, lol), and it won't be any fun for anyone else to read, either. It should be exciting - it should draw the player characters in and make them want to actually think through their posts and make the story exciting, not just spam it up with impulsive, poorly thought out posts. One- or two-sentence posts were actually grounds for banning on several RP forums I've participated in in the past, because they just annoyed everyone else and didn't actually accomplish anything. Give it some depth - your character isn't two-dimensional, their actions shouldn't be either.
One of the greatest qualities of a good roleplayer is patience. In an online text-based RP, one must be willing to wait up to several days for the story to advance even a few minutes. Although this can be infuriating, it also has a valuable aspect to it - you have much more time to think through your post and decide how you're going to respond to any given situation. Patience also manifests itself in another form - remember the realism? Action does not begin right away. Patience can also manifest itself in the willingness to participate in story and character development instead of constant action. Your profile is only the beginning - stay true to your character. How a person acts in everyday situations can let others know an astonishing amount about them - so be willing to develop your character outside of combat. The story may not always move at the pace you want it to, and attempting to force it disrupts things for everyone. Patience is a virtue, and especially so in online text-based RPGs.
This is also a problem in many RPers. Even after the initial problem of godmoding has been solved, many players have trouble remembering that their character does not know as much as they do. Remember to stay in character - your character would know only what they have been exposed to or observed themselves. In many RPs, players tend to forget that their characters cannot read others' minds, as well. Oftentimes, this can lead to conflict, as one PC may reveal their thoughts in order to add drama or tension to the story, and other PCs may react as if they know these thoughts, when in fact it is only the players themselves that do. Remember to stay in-character - assuming your character knows everything you do can be extremely disruptive to the flow of an RP.
Passive Godmoding, Part II:
The other part of passive godmoding is making your character invincible. Remember, in real life, people get hurt. They get hurt in RPs, too. Your character should not be a god - it should be a dynamic human being who grows and learns as it progresses through the campaign. You will not be invincible or all-knowing right off the bat, and it's important to rememeber that your character getting injured is not the end of the world. Sometimes, your character may get hurt, or even die - this is all part of the game. It does happen, and one of the greatest problems in the RPing world are those who try to overpower their characters...as usual, keep it realistic. Real people are mortal and have flaws, your characters should too.
Doing Your Research:
This is very important. When participating in an RP, if you don't know exactly what's going on, make sure to read the entire RP (at least to a point where you know what you missed) so you can stay accurate to the story. Oftentimes, when this doesn't happen, the story gets twisted or distorted, people get confused, and on occasion, entire RPs can crash because people lose track of where they are or where they're going.
Remember - even though you may not know what's going on, your character is there, and as such, should have a pretty good idea of what's happening in their world. Keep this in mind, and remember to do your research before posting. Off-topic Communication with others
If the GM requests a hiatus or a brief break in playing, do not publicly respond with your acknowledgement. Simply not responding is not acknowledgement enough, and if you must signal assent, PM the Game master to let them know you received their message. In the interests of keeping the game uncluttered, always remember to keep Off-topic speech to an absolute minimum. Also in this pursuit, if you must communicate with another player outside of the actual game - whether it be a question, a suggestion, or any other sort of off-topic speech - PM them. There are several RPs already with entire pages of nothing but off-topic speech, when an RP should have no more than one or two off-topic posts in a row at the most.
Off-topic speech, while one of the trickiest bits of RP etiquette to observe, since it is not actually clearly defined, can badly disrupt a game - as stated above, if you absolutely must communicate with someone about the game outside of your actual character, that's what PMs are for.
Another idea - if you need to communicate with everyone at once - rather than making a new post entirely devoted to off-topic speech, simply edit your last post and tack whatever you need to say on the end of it, clearly labeling it as off-topic when you do so. The aim of all this is to streamline the gameplay, so if you must make an off-topic comment, try to do it without disrupting the game, and as unobtrusively as possible.
Keeping The Story Intact/Splitting Off:
A pattern I have noticed among RPs on this site is that small groups of people tend to split off and play out their own miniature dramas, disregarding the rest of the players, and indeed the game, entirely. This should not be done - ever. One of the prime appealing factors to roleplaying is a coherent, continuous storyline. With this in mind, when playing, keep in mind that there are other players, and that their characters are living in the same world, at the same time, as yours. This ties into the patience angle as well - while it is extremely tempting to begin a duel or conversation with someone you know is online, this greatly detracts from the storyline as a whole, and is thus something to be strenuously avoided. Remember: In an online RP, patience is key. Make your post, and wait for all others to respond - not simply your target. Giving the others in your group a chance to play just as you are is half of what makes an RP fun for a group of players, rather than just a select few. Keep the story intact, and maintain your patience - it is okay to wait a day or two for all the others to make their move, as a story that takes its time in forming is that much better in the end.
Patience, Part II: Just posting your profile in an RP does not mean you should start posting immediately. Take turns - everyone will not necessarily move at once, and your character should enter in a manner that makes sense - wait until there is a clear opening for your character to enter the story, and then begin posting.
Godmoding, Part IV (NPCs): Introducing and playing NPCs (non-player characters) is the sole responsibility of the GM or GMs. Unless given license to do so by the game master, remember that you only control the specific character or characters that you are playing - all others are introduced and controlled entirely by the GM, unless specifically stated otherwise. Quality, Not Quantity: The point in an RP is not length - it is quality. A long post does not necessarily equate to a good one... the ultimate aim is a tight, well-written story that grips the reader by the throat and never lets up, dragging them along behind it like a runaway rollercoaster. If one or more of the characters go off into long, rambling monologues in the midst of a gripping, swiftly moving action scene, you're going to lose your readers' attention - this is counter to your ultimate aim. Remember, an RP is not merely a game - it is a novel simultaneously written by many different people, each with their own values and contributions. If these contributions are not woven together in a neat, tightly written format, the tale you are attempting to weave will fall apart. Just as a tapestry is composed of many different colors, an RP is composed of many different authors...and no one of these authors should ever overwhelm.
Remember - the ultimate goal is the story, not your own personal desires.
We don't do the whole GameMaster thing here, but who knows...it might be better if we did... I personally like playing the actual game to be able to sit and watch others play. XD