Minecraft wiki entry for Development_versions, associated text: Hunger
Sprinting
Activated by double-tapping W.
Experience
Currently has no use.
Different sound, more like a pop.
No sound for leveling up, (it doesn't really exist).
Blocking
Activated by using a sword.
Negates 50% of the incoming damage.
Critical hits
Does more damage depending how close the player was to the ground while falling.