WoW lore can certainly be used to more grimdark ends. The lore is toned down for a PG-13 audience; but politics, exile, war, death, destruction, those can be heavy themes if you begin to dig a little. Violence is VERY common in Warcraft, and I don't see the problem with more gray characters - this is CotH, we shape our settings to our tastes, and if we diverge from Warcraft's original setting, so be it. We play in a setting but aren't -bound- by it.
It isn't hard to imagine the most horrible things happening in many of the events of Warcraft lore, really. Wars everywhere, lawless areas...mothers losing their children, peoples being destroyed, soldiers coming home destroyed and with severe PTSD... and even in the most sophisticated of societies, spots of shadow (think of the rather authoritarian Sin'dorei - they're coming better, don't you think some people are gonna have trouble relinquishing their power?...Or Stormwind itself, which might be guarded well, but a wrong turn in a backalley and things can go south easily, and you can lose your life belongings, and end up homeless, or soemthing..stretching it a bit, but y'know)...
What about DKs? Forsaken? Dealing being utterly defiled, with becoming killing monsters, and being shunned by everything for something they just can't help..what about gnomes, ...oh yes, we see them as those cheery, bouncy things, but they certainly lost a lot of folk, and even their very -home-, or Tauren, those cuddly moocows, who have been saved from genocide.... and the list goes on, with most races having lost something of value, and most characters likely to have been hit by war, violence.
You will notice I am not limiting adult themes to sexual content and drug use (that's the commercial use of the term). -Many- other themes can have a dark or frightful tone. I'm giving some examples that might seem light to some, but go tell a mother about losing her child, go speak to those thousands of displaced people around the world, you'll understand things can become extremely dark and serious even with situations that really could happen every day. (
http://tvtropes.org/pmwiki/pmwiki.php/Main/AdultFear see this for instance - it doesn't have to be fantastic, 18+, or overly unlikely to happen to become a serious, adult topic)
My point is, even the cheeriest parts of Warcraft lore have material to explore for darker roleplay. Now, do we HAVE to play darker? Definitely not. But while all these happenings are toned down and shown in a cartoonesque light, I am fairly convinced many more adult themes can be delved on the existing lore, and if RPers want to play around that, who are we to stop them? After all, there are MANY others things that are left to be determined by players in the Warcraft world.
It is maybe mood-breaking in regard to the lore as a whole, but definitely not "lore breaking". I would welcome more adult RP (not that kind, no), though if people want to keep to the more cartoon way to interpret the lore...that's fine too. But I think adult themes are something about the lore worth exploring, which is not explored enough for the sake of PG-13, but we are CotH, we can allow ourselves to where Blizzard just can't.