2. Thou shalt not lay claim to spurious connections.
Your character is relatively far removed from the main characters of the official Warcraft games and novels. Although some degree of loose connection is permissible, any sort of close relationship is highly unlikely and unbelievable. Your character should stand on his or her own as an individual with his or her own story to tell -- relying too much on previously existing characters will make you look like a Mary Sue.
3. Thou shalt not strive to be the constant center of attention.
Everyone, including you, deserves his or her turn in the spotlight. Different people prefer different levels of participation of course, but as a rule, if you find yourself striding straight into center stage whenever you log on, chances are that some will start to resent you for interrupting their stories in progress. As in real life, listen before speaking, and speak only to those who are listening.
4. Thou shalt not strive to be the center of the universe.
We all long to save the world from the forces of evil, and to a certain extent there is room for that in WoW, through participation in quests, battles, and dungeon adventures. But the fact is that this still doesn't make your character more important -- many others are participating in these activities too. Let your roleplaying be about who your character is, who his or her friends are, and how they all interact with one another; not about who's got the longest title or most impressive storyline. Don't try to compete with Mary Sue.
5. Thou shalt overlook the inconsistencies of in-game reality.
When you kill monsters, they don't die forever. After some time, they come back to life, ready for you to come and kill them again. Likewise, if the monsters kill you, you don't really die either -- after just a few minutes, you raise from the dead, heal up, rebuff, and try killing them again. This and other problems require us to think differently about time and causality in WoW: you may invent story-driven means by which such game mechanics can be explained, or you may conveniently ignore such inconsistencies and impossibilities. Either way, focus on the relationships between you and other players, and don't sweat the small stuff.
6. Thou shalt remain consistent with the Warcraft lore.
Your character was born and raised in a fantasy world you did not create. Blizzard's Warcraft lore is explained at length in their Warcraft encyclopedia, timeline, and explored at even greater depth in fan-sites like WoWWiki and Dramatis-Personae. Take the time to read a little about the background behind whatever elements you wish to use in your character's story, from basics like race and class to complexities like his or her birthplace and specific events of importance in his or her life. Be sure to consult with a wise and experienced roleplayer about your ideas to help make them as good as possible.
7. Thou shalt make thy character memorable.
As important as it is to stay true to the Warcraft lore and avoid excessive melodrama and attention-grabbing, it is equally important to avoid blandness and trite imitation. Do not merely fulfill people's expectations for your race and class -- think of some interesting qualities and traits based on real human behavior, which make your character as entertaining or thought-provoking as he or she can be.
8. Thou shalt not impose thy style on others.
Your character traits may be designed for humor, or for drama, for casual fun, or serious storytelling. Roleplaying styles vary widely from player, even as each person's motivation to roleplay in the first place may be different. No matter how good your roleplaying is, there are bound to be some who do not enjoy it. Do not blame yourself, or them, but tactfully allow each person to do his or her thing. If no one seems to like your ideas, or if something becomes a cause of disunity in your group, seek the counsel of the wisest and most knowledgeable roleplayers you can find.
9. Thou shalt preserve the roleplaying server atmosphere.
Not everyone on an RP server likes to roleplay, but at the very least, everyone appreciates an atmosphere of respect. Roleplayers come to these servers for a certain purpose, and it is the duty of everyone there to respect that purpose. Therefore, the public /say channel on an RP server is not the place to discuss "phat lewts" or "relogging" or "dude did you see that movie lol." /Say is for in-character dialogue only -- all out-of-character conversations should take place in one of the other channels (such as /party or /whisper) which are more suited for it in any case.
10. Thou shalt strive to be inclusive.
There are few enough roleplayers in WoW that we must work hard to stick together, and make special efforts to be open and welcoming to newcomers. One of the things roleplayers love most about RP servers is a a special atmosphere that can arise when as many people as possible respect one another, behave with maturity, and help newcomers get involved and develop their roleplaying potential. Not every RP server has this atmosphere, but for those that do, it is because there try hard to create it.