I'm not informed enough about all the platforms out there to make calls yet.
I have considerable time in Second Life and a little in Quest Atlantis.
From there visited Reaction Grid, Jokaidia grid, and a couple of others whose names escape me now.
Necessary for adolescents: Persistence, Multiuser, Gaming aspects, WAN, shallow learning curve, Ease of communications in voice and text, creativity, Stigmergy, (if I understand that correctly), low to reasonable access cost.
I personally love rich graphics, but notice Minecraft's blocky environment is just fine with adolescents.