This page has a Mary Sue litmus test specifically for WoW RP characters. I took it for several of my characters and got really wonky results. (The higher the number, the more Mary-Suish the test believes the character is.)
MAELDAN: 0
OBSIDIA: 10
VILLESHARI: 13
SHELKAENA: 19
MEELLA: 19
ELLARCHA: 20
ZARCTICA: 23
That's pretty much backwards! My most indulgent Mary Sue-style characters got the lowest scores, and the most basic, ordinary characters got the highest scores. 0_o Makes no sense at all.
Maeldan is the long-lost illegitimate son of not one but two major lore characters, and dabbles in experimental magic and necromancy. In fact, he single-handedly brought a major lore character back from the dead.
Obsidia is the daughter of a Dragon Aspect, exiled and trapped in human form. Her sidekick is one of Onyxia's whelps.
Villeshari was a casual acquaintance of Kael'thas, and King Anasterian approved of her as a possible match for his son.
On the other end of the spectrum, Shelkaena has no connections to major lore characters at all. Her mother was a ranger so she probably saw Sylvanas a couple times, but that's about it. She has no deep dark secrets or superpowers. She's pretty average in most respects.
Meella was a novice in the temple that King Terenas and Prince Arthas attended but she didn't know either of them personally, and she doesn't have any special abilities.
Zarctica has the most mundane background of all. She and Jinzakar (who I didn't take the test for) were sisters living near Sen'jin Village in Durotar. Their father was a fisherman. When they got old enough they wanted to see the world a bit so they set out and did some adventuring. She likes flowers and cute fuzzy animals, and the Undead give her the creeps. She has no secrets, uber powers, tragic angsty past, or anything like that.
Ellarcha isn't very well-formed in my mind yet, but the thumbnail background that I've worked out for her is that she's related to Archimonde on her mother's side of the family (like second or third cousins, maybe, and she never really knew him). Quite a few of her family members sided with the Legion and became man'ari eredar. Ellarcha herself is somewhat paranoid about showing any kind of dark side, worrying that she's got some genetic predisposition for evil. Some days she overcompenstates by being obnoxiously pious, blessing everyone left and right and gushing praise for the Naaru. Other days she lets her guard down a bit more and casts Shadow Word Pain on squirrels when no one is looking. XD She's a good person deep down but she struggles to find a balance for herself.
So while the Mary Sue litmus test was interesting and entertaining to fill out, I'd say the results were way, way off.
Obviously there's a lot more to Maeldan and Obsidia's characters than their parental lore connections, but I'd still say they rate a lot higher on the Sue-scale than Zarctica and Shelkaena, whose backstories are extremely ordinary.