Minecraft has had a good ban system since before it was officially released. Servers could ban by UserID or IP, and I'm aware of at least one plugin that (among other features) allowed servers to join a ban pool and ban players that had been banned on other servers. This is a centralized ban system that is completely inappropriate for a game like Minecraft where servers are hosted independently. Different servers are going to have different rules to accommodate different age ranges, and most people will adjust their behavior to remain within those rules. Now if someone goes on an 18+ anarchy server and calls for the extermination of the Jews villagers (in Minecraft™) they can be banned from playing on any online servers.

It isn't any business of Microsoft's to tell users what they can or cannot do on servers they do not own or run, and using the excuse that "it's for the children" doesn't make it any more appropriate. The responsibility of ensuring that children are not exposed to inappropriate content falls onto their caretakers,, not Microsoft. If Microsoft wants to add an opt-in parental controls feature I'm fine with that, but not this system.