http://t.co/bsKW5CvB

Have fun.

Tried it out, its really buggy, for example when I tried digging or punching a tree I couldn't really move.My first person mode was focused on one block, even when I tried to look around. Yet in 3rd person it was showing me where I was going,etc.So it was okay, except the basic movements of the player are now bugged.
NOTE:This was a first impressions of the new pre-release.

P.S despite the bugs(and bug fixes) Notch said he added a few new things.

ChangeLog

• Block of iron texture is changed. Pic
  
• You now have mushroom biome grass in your creative inventory.
  
• Fence Posts no longer connect to leaves.
  
• More achievements.
  
• Annoying Stair glitch fixed. Confirmed by redditor meesebyte.
  
• Option to disable clouds in menu.
  
• F6 = smooth walking It seems to allow you to fly straight but not up or down (works on survival). It acts as a noclip, lets you go through blocks deals you damage... Your Y co-ordinate doesn't change.
  
• F9 Adds a fixed camera angle. ( You can move this with I, J, K, and L)
  
• Passive squid animation is now vertical. (Credit to B1ueStarfish for finding this)
  
• Lily pads are no longer gray in inventory, have a tool tip.
  
• In creative mode hostile mobs no longer attack you unless
• provoked, except for ghasts and spiders. Note Enemies from spawners are still hostile. (Thanks theriverotter)
  
• Melon slice drops are back to normal! (Thanks dg200)
  
• Pressing "N" "M" "Y" "U" "O" and "H" changes depth of field and angles of camera. (Thanks Pupetteer)
  
• You receive a warning when changing render distance to far.
  
• Items retain their enchantments if chest is destroyed.
  
• You can now look around without a mouse using: I, J, K, and L.
  
• Slimes not restricted to y0-16 anymore.
  
• Game Crashing piston glitch fixed.
  
• Player bow drawback animation improved.
  
• Silk touch does not work on spawners.:(
  
• Instant Heal splash potions damage zombies.
  
• Vertical Water physics are different. (Not sure if bug or otherwise)
  
• Cool Enderdragon Death Animation.
  
• Snow Golums visible!
  
• Longer reach for mining as well as attacking.
  
  Possible Changes
  
• Grass seems slightly different.
  
• Overall performance seems to be better, less resource intensive.
  
• Longer reach for fighting mobs. Confirmed
  
• Terrain generation seems to have been tweaked.
  
  Bugs
  
• Fences don't keep animals in effectively.
  

There are more bugs then fences.

- Known Bugs