Beat me to it!
This is the pre-release; Jeb leaked it deliberately.
I think it is a pre-release because 'they never release on a Friday' - there will be nobody around to tackle bugs over the weekend, but now we have a chance to find them.
I've been playing this for about an hour now, and wow. I think the way we work as a community will really change now - biomes are absolutely huge:
Forests: can go on for miles, and are made up of a certain type of tree (ie. evergreen forests, tall tree forests).
Oceans: They are absolutely huge. Ridiculously. Even with no view limit on, I couldn't see from one side of an ocean to the other. It would take a long time to get across one.
Rivers: Not seen any yet!
Mushrooms: Not seen any either.
Endermen: Aren't actually that rare at night, in this version at least. Rarer than other mobs, but not difficult to find. They are pretty strong too - as expected they only attack when you look at them then look away, but there is NO outrunning them, they can teleport THROUGH things (so no hiding in a building either) and teleport behind/to the side of you if you try and attack them!
Their one weakness: Water. It hurts them like fire does to other mobs.
NPC Villages: I was in creative mode when I found one, and couldn't see any people (Notch said there wouldn't be any). However I heard someone mention that there were in fact NPCs in the villages, so I don't know. No village boundaries that I can see.
I can't see villages being all that useful right now, but I can see them being used by newbs who have no idea what they can do in the game.
Swamp biomes are pretty cool.
The only issue I've had so far is that the first time I loaded it up, the game put me on a tiny desert island with one tree right in the middle of an ocean. I couldn't see any shore in any direction.Edit:
BLUE SPIDERS - 1 block wide, they can get everywhere!
Abandoned mines are also pretty cool