Despite the title, I have not seen what I would consider graphic violence in the game. And I'm someone who's pretty squeamish about violence. The main thing you want to consider when determining whether or not your child is old enough to play this game, is that you do in fact kill things. It's not as if you see a mangled corpse, but your enemy does lay on the ground, unmoving.
Given that this is a MMORPG, you do encounter other players in the game, and it goes without saying that they aren't always pleasant. You can, however, turn off the chat feature in the interface.
It is true that not all characters in the game are modestly dressed, but I don't see this to be at all a problem. To quote Liz Lemon, nothing in the "bathing suit area" is revealed. It's not as if the characters are oozing sex. They're animated characters and I personally don't see the big problem if their shoulders or stomachs show.
This isn't a great game for tweens that are easily frightened. There are zombies and monsters and the like. But again, nothing particular graphic or in your face.
Long and short of it: it's fine for kids on the younger side provided their parents know about the different safety features and tools to limit play time. (Visit battle.net for more info.)
Oh, and I do think there are more lessons to be learned then what's stated. You can talk about the warring factions and whether they consider one to be the "good guy" or "bad guy." Discuss the points of view and the good and the bad in both sides. There are some good role models, even in war. Lady Sylvanas, who freed herself and others from oppression and leads them with the hope of one day freeing all of her people. And other great leaders from both sides. Another good topic are the outcasts. The undead and worgen (werewolf like creature) races are two sides of a coin, most of them once being human and now looked down upon by the sides they fight for.
As others have warned, this is not a game for people who have what I call
"addictive personalities." My dad bought this game as a joint present for my sister and my 17th and 18th birthdays. (We'd been playing on a starter account.) My dad will play for a couple hours after he gets home from work. My sister will also play for a couple hours a day. I generally play less than an hour a day. I find the game is more fun when played for short periods at a time. It's a good stress reliever. Too much play time brings stress, however.