The problem with letting kids play these games is simple.. although they have nothing really to do, growing up on these games is like growing up in a seperate reality. ESPECIALLY mmorpgs like World of Warcraft, Everquest, etc..
The main problem they face later on is the near total lack of face to face people skills, and the inability to relate to anything other than the games they play. The people I have met in real life that iv'e known in game for a while end up being totally different face to face.. it's almost as if they can't cope with social interaction unless they are doing it through a keyboard and monitor. The fact that games are violent only augments this effect, but not to a point where it will cause someone to go on a shooting rampage at their school.
If you want to pinpoint the problem here, all you have to do is take a step back and look at their parents. I don't care what anyone says.. the parents are what mold the kid into what they are. If the parents know what they are doing, they'll make sure that there isn't a path the child can take other than a good one. If they don't know what they are doing, they'll force the kid to go one way which will only make them rebel and choose foolishly.