Don't: Stick to the same select few. It's nice to have a group of reliable friends and guild members, but cutting yourself from everyone else entirely isn't healthy. Be prepared to meet new people who are likeminded!
Don't: Get upset when your character takes a bad turn in an IC fight, whisper the person and come to an agreement over it.
Don't: Travel to Silvermoon to Booty bay in one hour.
Don't: Be shy to roleplay whatever you want. People are stupid and will hate you no matter what, even if you haven't provoked it, so what the hell. May as well enjoy yourself.
Don't forget, you don't have to conform to what other people think you should be doing in roleplay. We're all human here and can be wrong. All I'll say is, that if you opt to not conform, do no be surprised if people lash out at you. It's jealousy half the time.
DON'T: Be afraid to step out of the racial stereotype. It seems to apply to gnomes more than most. There's oodles of fun to be had being different. A word of warning though, know what rules you're breaking before you break them or you might be heading for a world of pain when the RP Police catch up with you!
Don't: Change large parts of your character's personality or background purely for the sake of joining certain role-playing guilds. If you're character is originally from Gilneas, stick to that instead of suddenly having them spawned in Lordaeron instead.
Do: Be sure to cling to the base traits of the race you're representing. An elf is still very different to a human even if they've lived alongside each other for a few years, after all.
This. So, so much this. I've seen people try to play their Elves running along a similar concept to Sav, but they end up playing their Elf as an Orc. By all means, play an Elf that's been alienated from their own society and as a result become attracted to the 'freedom' that Orgrimmar and loyalty to the Horde offers, but don't forget that, no matter how alienated your Elf has become, they're still going to have some ingrained Elven traits. Sav, for instance, has an instinctively fierce sense of self survival, which is something I most definately see as ingrained into the Blood Elven mindset. This instinct comes above honour and loyalty for him. He's cruel and ruthless, but not in the sense that an Orc would be. An Orc respects a worthy enemy, and honours him after they're defeated. Sav butchers the enemy and spits on their corpse. Just something to keep in mind for those who, like myself, are playing an 'Orgrimmar-Elf' concept. Your Elf is never going to quite blend in with Orcish society, not really.
And as for some more Dos and Don'ts....
Do: Keep in mind the logistics of a battle during RP-PvP. You cannot, realistically, just walk straight through someone. If they've got a solid line set up, there's no way in hell you're going to break through it unless you physically shatter their defenses by taking down some of the soldiers in the line. You can't just fight through them and get straight to their menders. This especially applies if you're a rogue. You can't just go invisi-mode and sneak straight through them to magically reappear on the other side.
Don't: Be afraid to ask for advice on a concept if you need it. If you're unsure of how something will be recieved, ask your guildmates, or even someone on the forums. But be prepared for some rather frank criticism. If you know you're going to be hurt by someone saying they dislike your concept, then it's probably best that you -don't- ask for help/advice on it.
Don't: have too much scars you're not a bleach charecter....oh and dont wear a full plated armour attire with a snazzy and light hat on top...
Don't: jump blindly into stuff you don't know or have experience about, common example, demon hunting.
Don't: Attack "I'm not a DK" characters for wanting to roleplay what they want with what they have, and another reason I've seen thrown at "I'm not a DK" characters is that they should play their class ICly, the reason they don't is because Death Knight's have nothing to role play any more.
And another don't: Read the book before you judge it.
Don't: Fear us. By all means come on over. We need more people to play hangman with us.
Don't: Kill another's character without permission as it isn't right towards them, also remember that only around 4% of the pop in WoW actually reads the forums and can't know about all going in here such as locations and whatnot so be reasonable.