I have a ten year old son ergo minecraft exists in our house. He's heard of it and played it a few times in the last few weeks. It seems to be a bit of a primary way of interacting among geeky boys his age, which, lets face it, he is.
My concern is actually a fairly straightforward one. The horror tales I hear are of kids neglecting basic human needs-interaction, exercise, food etc - in order to play this devilishly addictive game (to be fair, it does seem quite compelling). OTOH, I grew up with computers and programming them and I remember a similar moral panic over things like SuperMario and Donkey Kong which, looking back, seems misplaced. The kids who grew up playing Nintendo generally I think are not still playing Nintendo.
I don't have too many qualms over whether its educational. I'm sure it has an educational value, and that's cool, but a bonus really. Educationally, I'm happy with the amount he gets up to. Regardless, a kid needs downtime and I'm ok with that being on a computer. The issue I have is that it is primarily solitary (in terms of face to face interaction) and sedentary. These are kind of the reasons my kids aren't in school so I'm not keen to replicate them by handing them an addictive computer game-if that's what Minecraft is!