Too many mindless MMO, that is. I've yet to see a MMO concept that could appeal to adults and not to some asocial teens.
The existence of countless adults in World of Warcraft and EVE kind of contradicts what you see, Morgoth.
Being adult doesn't mean grown up. WoW doesn't use any adult or compelling themes. It's just, as theslug pointed out, a waste of time and money without any intellectual reward.
You forgot to address EVE.
Indeed. However he chooses to define "adult or compelling themes," he'll have a hard time excluding EVE with its extensive politics, espionage, betrayal, and space piracy.