Trust the Minecraft community to make a mod of a mod. In this case they're currently building a version of DayZ, the brilliant persistant Arma II zombie mod.
The more you think about it, the more sense it makes: Minecraft also allows PvP on a map with hostile NPCs, it has hunger, loot, a day and night cycle. The devs are merely guiding it to its destiny with MineZ.
They've hand crafted a world with plentiful supplies near the south coast, where the player spawns, and better gear further away to the North: this is not about exploration for the joy of it, like in vanilla Minecraft, you'll need that gear to take on the players and the zombies.
The NPC threat is a bit different from the usual Minecraft: Zombies have been tweaked to be more threatening: they can detect you according to how loud and fast you are: you can actually sneak up to them, although they also have a line of sight as well and can even see through glass. Sprinting will cause them to give chase, but with a moving speed set slightly slower than your sprint you can outrun them. Even if you do, though, they'll hunt around the last area they saw you in. Your UI will tell you how visible you are, but who has the wherewithall to check it when you're being chased?
All the while you're getting hungrier and thirstier.
Resources are controlled as well: you'll either need to craft food and equipment, or come across a rare chest that'll contain interesting bonuses. You can't mine in the usual fashion here, so scavenging is your only hope. Shops contain susteneance, barns and workshops the bits needed to make tools, and military bases spawn weapons and rarities. There's a sliding scale of complexity involved in getting at them: the military bases can only be accessed through a series of buttons and levers.
Like the DayZ, it's a persistent world, and your decisions follow you from server to server. I've already taken to joining hardcore Minecraft servers when DayZ fails to allow me access; with this mod coming in a few weeks, it might be enough to yank from Arma II's addon altogether.