Minecraft + MMO would be awesome, but I would wager it would be virtually impossible with current server technology. Or rather, it would be far too expensive. In addition, MMOs require a certain level of structure. If they wanted to allow us to sculpt our own lands, then what's stopping someone from smashing down Uldah? If someone builds a new and better town, what's stopping people from smashing that one down too?

I really wish MMOs would do more things with housing and maybe instanced outdoor gardening, but as far as public terrain goes I would say keep that part out of the MMO genre. Leave it to games like Minecraft and Terraria, they are quite successful in the respective genres.