While I do respect your argument, the whole thing hinges on the one fact that every player who plays or experiences Warcraft has to have preconceived notions about the nature of Orcs, Minotaurs, Goblins, Trolls, and the Undead. If such was the case, and such exposure was required to constantly remind people that Our Orcs Are Different, then you have a point.

The argument starts to fall apart when you start to consider that, maybe, someone doesn't have preconceived notions about Orcs, because they never played D&D or they didn't care much about the Orcs of Tolkien's writing (or maybe they've just never read the books or seen the movies, which is still possible). Maybe someone honestly has no true idea what a Minotaur is and why they're different from the Tauren. Or maybe someone can already tell by their appearance alone that the Goblins of WoW aren't going to be at all like the Goblins of Labrynth, so they toss out any preconceived notions at the very start on their own.

Once you remove that requirement, that people have preconceived notions about the Horde races and need to be constantly reminded otherwise, then all the focus on the Horde does come off as favoratism and bias. It comes of as Blizzard constantly focusing on a single aspect of their universe and neglecting to give the other side the same treatment.