It is an art to turn any negative situation to your advantage, and no less so when roleplaying in WoW. In the fine tradition of "turning stumbling blocks into stepping stones," it pays for a roleplayer to reconsider a number of in-game situations which seem to get in the way of roleplaying, yet which actually offer a special opportunity to showcase your creativity.
The biggest stumbling block WoW roleplayers trip over is often some aspect of the game mechanics themselves. Your roleplaying may lead your character into a deadly conflict with another player, for instance, and yet even if you kill the other in a free-for-all PvP arena, he or she can just resurrect and be back to normal in a few minutes. Alternately, you may find an epic BoE drop off a Skettis Kaliri and be hard pressed to explain how a rainbow-colored owl was flying around with a huge sword inside its body. You may even ponder why every single ogre you've ever seen is male.
Naturally, of course, there are ways around all these problems -- it's just a matter of finding plausible reasons for things. You may say to your bitter rival, in the event of a deadly conflict: "I do not kill fellow members of the Horde! We shall duel for honor and be done with this!" Likewise, when recounting your discovery of your BoE epic sword, you might explain: "As I killed the strange owl, I suddenly noticed something gleaming in the grass just next to its corpse! This [Blinkstrike] was lying there, sticking out of a stone in the ground!" Your character might even make an effort to explain away in-game oddities: "I have deduced that the entire race of ogres must be hermaphrodites -- both male and female at the same time! They are so ashamed of this that they all hide the fact, pretending that ogre females are hidden away somewhere!"
Solutions to such problems tend to be best when they are at once intuitive and plausible. Even a far-fetched explanation or idea can work if it really fills in the gap between what can happen in the game and what should happen in our RP stories. Mileage may vary, of course: what strikes you as a funny and plausible explanation for something may come across to others as rather contrived. Nonetheless, overcoming such problems of game mechanics in roleplaying needn't bring you down -- in fact it can be one of the interesting things that you love about the game, an ever-present challenge for you to test yourself against.
There is a tendency, for example, to completely separate in-game fighting from roleplaying, or to simply ignore the ambiguous game objectives while roleplaying. But, while generally the "heroic" stuff is merely a background for RP, it needn't always be so. Earlier today, I asked one of my friends to join me in killing some mobs for a little while in order to get the last bit of reputation I needed to reach Revered with Keepers of Time. Instead of using this as a rather bland motivation for my character, however, I decided to make it a search for a stolen item instead. My draenei hunter and her friend would go through the Caverns of Time, searching out which of the Old Hillsbrad guards was an impostor from our own time -- a thief who had stolen a magical crown and escaped into the past with it. Whichever mob was the one to bring me to Revered would be the one who happened to have the supposedly stolen crown I already had in my inventory. Thinking of a more interesting and plausible reason for our characters to be doing this helped make our time together less of a grind and more of an immersive story.
In order to make roleplaying work this way, the greatest rule is to be flexible and balanced in your approach. Your solution will often rely on some creative use of abilities that isn't actually what's happening in the game. This draenei hunter of mine, for example, has her own understanding of the Light, and she has this idea that she should never kill any other living creature. While "Tranquilizing Shot" only removes "frenzy effects" according to its tooltip, from a roleplaying perspective, it can also help explain why my character's enemies appear to fall over and die, when actually I'm just hitting them with a tranquilizer.
While such bending of the rules can work very well for existing abilities, some roleplayers may be tempted to use spells and abilities that don't actually exist in the game. I met one such roleplayer who had invented a whole system of "spellsong" magic for her mage character, which supposedly enabled her to change from a human to a high-elf and manipulate the fabric of reality, among other things. It was truly creative indeed, but it didn't seem to fit with anything I could actually see, or any of the lore I already knew about the game or the Warcraft universe.
Always, using an existing spell or in-game visual effect in a new way is infinitely preferable to just typing out with emotes something you made up. I once met a gnomish mage who was very much "Frost" specced for RP -- she walked around all the time with a perpetual head cold. Sometimes she would use a macro so that whenever she sneezed, she would unleash a Frost Nova just as she said "ACHOO!" She was very much the center of attention wherever she went, and the way she made the game mechanics work for her earned her lots of respect and made everyone around her laugh.
Roleplaying in WoW is about making the the game, the world, and the lore work for your storytelling and character development rather than against them, even in situations where they might seem to be an obstacle. Think of everything in the game as elements in a kind of rubix-cube Jigsaw puzzle, waiting for you to as you arrange the pieces in the right way and let your own characters and stories take shape. Don't try to squeeze your own extra pieces into the mix, but instead go all out in making the existing pieces work for you in your own creative way.