How to RP a powerful character without making other RPers hate you
Playing a powerful character in a static game like WoW is difficult in many ways, which is why many kids or people not familiar with RP tend to get it very wrong. However, although it might be easy to dismiss anyone playing a powerful character, it is possible to play one and do it well.
Earning your character's power
Power doesn't come out of nowhere, and there are many kinds of power out there in the Warcraft universe. Is your character a powerful warrior or mage? An aristocrat who is well-respected by your people's government? A person with a unique talent?
Earning power is another thing you should think about. If you go for a "powerful warrior" kind of character, you should probably take your character's level into account. While many roleplayers argue that levels and other game mechanics have no place in RP, this automatically makes any of your in-game PvE and PvP activities out of character, and it will be impossible to roleplay encounters that are normally suited for characters far above your level directly in-game. No power or skill comes without an effort, and combat skill is no exception.
- You have the complete agreement of the character you're emoting to.
- There's no one in the immediate vicinity who might take notice and interfere/interrupt.
- It's somewhat believable.
Still, since RP is about improvised in-character interaction and not something that is scripted in advance, it is preferable that all IC conflicts are carried out in a way that makes all characters involved able to resist or escape.
Powerful beings take notice of other powerful beings. It's a fact. So, if you intend to wander around and be blatantly powerful, then you'd better have a fantastic reason why. Say, SI:7 hasn't assassinated you for being a huge threat or the Burning Blade hasn't tried to turn you to their cause. You also need to think about other players - how are they going to react to your power? Because of the game mechanics, you can't just squash them if they get in your way unless they agree to be squashed, and if you're going to just emote at them and walk away then you've failed as a god-character. You have to be believable above all else, which means thinking objectively about how the world reacts to you as well as how you react to it. If you choose to remain hidden, then think about why your character doesn't use his powers - is it in character for a dreadlord, say, to disguise himself at every opportunity, even when he could squash the people around him without repercussions?
Absolute power corrupts absolutely
Perfect, overpowered characters are quite simply boring as hell. If you can do anything by snapping your fingers, then why bother doing anything at all? If you can deus ex machina your way out of any situation, then your RP will get stale very fast. So playing an omnipotent character is pointless, and playing a powerful character without any flaws or worries is almost as pointless because there's no room for the conflict that leads to good RP.
Take Superman for example. Super strength, lazerpewpew eyes, X-ray vision, invulnerable, and on top of all that he's a super good guy. How do you write a story for such a two-dimensional character? They had to invent kryptonite just to have something to base a plot on that wouldn't bore readers to death. Now compare him with Batman - a character with no super powers, who isn't always good, and who has some horrible emotional scarring because of his parents death. He has far more room for good plots and stories precisely because he isn't as powerful or as perfect as Superman.
Respect the lore
Since you are roleplaying in a universe made long before you entered the game, you will have to respect and follow the lore that has been already established in it. If the background of your character contradicts some known facts of the Warcraft lore, you will most likely end up ruining the fun of the players who are familiar with these facts.
That said, you can certainly expand into areas that are not described. If the lore says nothing about the average human diet, then you can be vegetarian and make that a part of your character. But this is not a reason to just shove anything you like into WoW - just because it's never said explicitly that orcs don't carry AK-47's doesn't mean that you can RP a hunter with one and get away with it. It has to fit the fantasy setting - so you can RP an engineer or mad scientist who's come up with some new kind of rocket, etc... The point being that if you want to add something new, then it shouldn't seem completely out of place. Another example of the things one should avoid is inventing places or races that were never mentioned in the known lore (such as new worlds and new continents). Such things are better left to Blizzard.
Plausibility is, of course, the most important thing if you're going to expand a part of the lore and add something new.
Demons, angels, vampires, dragons, werewolves
Everyone likes to play the clichéd characters. All of the above have been done to death and then some in books and films, and it's to be expected that people will want to play the same characters in WoW - hence the large number of Legolas-wannabes. It's easy to fall into the trap of being a copycat, but hopefully if you take the time to develop your character you can avoid it.
The best way to avoid the clichés is to take a typical one and twist it or change it into something weird. There are no vampires in WoW, but if you want to play one and not turn into the latest clone from an Anne Rice book, then use the world to make it different. For example, the Scourge are all about developing new plagues - what if they tried to make one that was supposed to cause a human's blood to decay and rot inside their bodies, but had odd side effects like adversion to sunlight and a massive thirst? Or magical side effects like superhuman strength for a short time? Just about anything that can fit into a fantasy setting can be added to WoW as long as you're willing to adjust it to the lore - be very wary about bringing something verbatim from another setting, because chances are it won't work. You can't just say you're a Jedi, for example - but it would be perfectly acceptable to write a backstory for a group of draenei mystics (mages/paladins?) who are known for using a particular kind of sword, having magical healing powers and go around settling disputes and being all Zen and whatnot.
Patch 3.0.1 edit: Darkfallen are currently the closest World of Warcraft equivalent to Vampires, but most charactaristics following them is that most of them are Blood Elves who died in assaulting the Frozen Throne under Illidan's command.
Patch 4.0.1 edit: Also, Worgen.