The open forum on the success of Minecraft versus SL has attracted some really interesting reader comments, one of my favorites being this one from Jo Kay, an educator deeply involved with Second Life and OpenSim as a teaching tool:
One of the key factors IMHO is Minecraft's simplicity AND its complexity. A small child can quickly learn how to negotiate the space (without lengthy tutorials or clunky viewer software to learn) and get almost instant satisfaction from exploring and socializing, monster slaying and/or building a little house, castle, secret base etc.
Meanwhile at the other end of the spectrum, Minecraft allows for complex modding which has seen developers from around and world and across a huge age range creating their own versions and flavors of the game - from themed servers (Hunger Mines, Harry Potter etc) to Factions, Feed the Beast and PVP).
Additionally the extensive "metagame" that exists around the game is compelling - from YouTube stars to kids writing epic Minecraft theme stories and beyond, the eco-system of fan created media is endless and amazing and operates from beginner to elite. Kids are able to join that community easily via Twitch, Youtube, fan sites and Forums, and via coding communities like Bukkit.
... all of which leads to several advantages of Minecraft as a pedagogical platform: