First, I think Blizzard has made the games where most gamers have played someone not White. Most protagonists in video games are White (not including sports games or the Sims since those are more simulations and less fictional narratives, so I seem them as a separate but related thing). The way I see Blizz having bucked that trend is not through direct racial representations (like the WD), but through playing up some of the racial allusions embedded in fantasy archetypes. And I'm primarily talking about WoW here (sure, the paladin was Black, which is an interesting switch especially in light of the role of Christianity in Africa and the African American community, but he was mostly a holy crusader type and not so interested in playing out or upsetting racial narratives). The subaltern conglomerate know as the horde brings together Blacks (orcs, trolls), Indians (Tauren), goths/punk-!@# teenagers (Undead), and gays (Blood Elf males...females in this model would be gay groupies?) all under the command of the Orcs, a race uprooted from their tribal/shamanistic existence by a foreign power bent on conquest and looking to use the Orcs as some free labor. The Orcs are now dealing with being freed from slavery (Twice! the story of Thrall as civil rights leader...Thrall = MLK and Grom = Malcom X? Malcom doesn't come off all that well here, prolly pre-Hajj. Or maybe its Dubois and Booker T. Washington...lol, no) and attempting to reclaim the culture their forced diaspora wrenched from them and negotiate their new, embattled place in the world. While this isn't exactly good history, it does play on many prevalent themes in the narrative of the African American community. Thrall has become one of the most celebrated characters in modern fantasy and the King of Stormwind is routinely criticized on these forums for being "xenophobic" (a term which includes racism while also opening up its connotations to the fear/hatred of ALL difference).
Now, the Witch Doctor. While his origins in an impenetrable jungle populated by tribes dedicated to ritualized blood sacrifices and other decidedly left-handed magical arts first conjure up visions of the African tribe in Conrad's Heart of Darkness, and all the colonial baggage that novel is dealing with, he's positioned in his short story as a cultural rebel. He's dislodged from a dangerous culture of Black alterity but uses its modes of reproduction (ritualistically mobilizing natural threats like locusts, disease, and herbal poisons along with iconic markers of voodoo like toads, chickens, fetishes, and zombies) to pursue his new path. He is at the same time a rejection and reinstantiation of racial narratives. He's not really on anyone's straight and narrow. His posture is stooped and he has a pot-belly, further differentiating him from the archetypal mode of fantasy hero as exemplary European demi-god. His body, so often the site of racial demarcation, doesn't even fit into these narratives. The necromancer, the class he's most compared to in style, is pale, straight-backed, proud, and thin like his European aristocratic model (kinda vampy). The Witch Doctor is nowhere near as familiar to most gamers/fantasy enthusiasts in terms of either cosmology or ease of identification with the character.
Some of the things I see falling out from this happen in both actual gameplay and expanded role within the genre for multi-ethnic representation. The gameplay, as so often noted by people excited about playing a Witch Doctor, is dynamic, flexible, and full of possibilities. I think this comes partly from the characters racially liminal position in the fantasy world. Its not tied down to archetypes (meatshield, rogue, caster) nearly as much. And this has the potential to be leveraged into a shaking up of the European based fantasy model in gaming.