I imagine a lot of roleplayers have done "playing as the monsters" at some point, even without taking into account things like World of Darkness, factional MMORPGs such as World of Warcraft, or of course fantasy wargames.
In a traditional fantasy setting such as the Old World, dark elves or goblinoids would seem the obvious choice for monster PCs, but the former have a distinct ring of Mary Sue to them, whereas orcs and goblins are too fractious and (dare I say it) unsophisticated to give much depth to the roleplaying or longevity to the campaign.
Chaos cults are another obvious choice within the Old World, and to some degree would suit a WoD "society with society" type campaign, but I'm not sure that offers anything particularly new.
What would interest me would be a "killing things and taking their stuff" campaign playing as Skaven, or un-reconstructed WoW Forsaken, where characteristics and motivation are sufficiently human while morality and quest objectives are diametrically opposed to the norms of the game. I wonder, having played such a campaign, how players would then approach a traditional D&D scenario once they're back in the shoes of the "good guys".