I really really wish they did something in multiplayer. As it is now you don't take damage from any source, and they can only be killed with fire. It's like cheating with god mode, and is lame. What would be awesome is having hordes of zombies come out at night to assault your position, slowly attacking walls and weak points. You and your allies are not able to place blocks at night, and must defend yourselves until the dawn comes.
I read on his front page somewhere that he is coming out with an Zombie apocalyptic mode along with CTF to expand multiplayer.
Today, I had an huge oh [censored] moment. I was digging a tunnel to my spawn. I was almost done, and was slowing digging my self up and out of water when I heard the "Ssssssssssss" I quickly ran back into down away from the noise and water to find out that two cows spawned right behind me. I couldn't move them and I turned around just in time to see the creeper, before it flooded a fair amount of my tunnel and left me running with half a heart left.
I cant imaging playing this game without them.