This is reported straight from unverified observations from the PAX demo Notch put up. Who knows what funkiness he has tweaked.
Oh right, I was going to finish that :V
PAX Mojang booth report:
Endermen are scarier than they looked in pictures. If you aggro one, it will slowly begin moving towards you(not entirely sure if this is true anymore after seeing some videos, redacted until further evidence is found), unless you look away, in which case it will teleport and speed up. Running is a bad idea. Presumably it will start hitting you if you sit and stare at it long enough for it to reach you. Haven't seen any drops. They burn in the sunlight. You will only ever aggro one at a time, the entire group will not start moving at you once you piss off one.
Zombies now drop Rotten Flesh, which will heal you instantly for some number of hearts, but will poison you, causing your hunger bar to decrease quickly for some period of time.
You now have to draw your bow, length of draw affects the arrow's distance flown but haven't tested. Length of draw also effects damage done. 10 arrows to kill a pig at minimum draw, 2 at maximum.
Arrows can be seen stuck in the mob they hit for a short while, after which the arrow vanishes. It's purely aesthetic, you can't get the arrow back.
World gen is more awesome (sweet floating islands(unless it was a fluke))
Title screen changed to something similar to Aether's title screen, slowly pans around a map (probably last one played).
Chunks appear to be cubes now, but not sure (That's how it looked when Jens was flying around in creative mode.) Redacted due to no solid evidence
The new lighting engine looks great, and things are brighter in general. Finding yourself in a pitch black cave with no ability to see may be a thing of the past, although with the new brightness slider you can probably just adjust it.
World seed is displayed in the F3 menu.
Naturally generating abandoned mines, possibly only at the bottom of ravines. Contains natural wood planks and webs. Didn't get too good a look at one of these, so there's probably more to them.
I think that's everything I noticed. I didn't mention thing we already knew about like windows and NPC towns. The towns were empty of inhabitants at the demo, so the NPCs are probably still really buggy. Also, the memory crash still isn't fixed. It happened on stage two or three times just while I was standing there.
I have over 14 .jars in my bin folder.
Two of them have original gameplay.
Might I suggest
Minecraft Portable? With this you don't need to move around .jars, you can just create a new install of Minecraft. It's a bit space inefficient (~40 mb of wasted drive space), but it's useful for keeping various mods organized. The only thing I've found that isn't compatible with it is the Yogbox, as that doesn't let you choose the install directory. Also, I've been using it for months with no problems, so I think it's trustworthy.