The multiplayer right now is pretty incomplete but kind of interesting.
Right now players aren't transmitted, but changes to the landscape are. So you see blocks disappearing/reappearing out of thing air. Monsters are all client side too, so you can't fight monsters with your friends. It will look like you're randomly taking damage for no reason. If the host gets killed at the moment the server crashes too which is pretty hilarious. In the stream the host got killed by an invisible spider. The game actually kept running without the host and didn't lock up until he tried to get a new chunk of the landscape from the host. Water and landscape changing does work already though, and he's starting with making infinite maps multiplayer, then working backwards to limited maps.
He mentioned a lot of stuff in the stream. There's a couple of lists on the minecraft forums. Off the top of my head there were road blocks, dyed clothes, backpacks and diving stuff.
Poll