From the /v/ topic today, these were discussed by people and notch:
Carryable light sources
Torches that burn out, Lanterns that never do
Multiplayer modes that can take a week in real time, with competitive goals and sabotage/raiding
Carts to carry goods over long distances
MINECARTS
Goblins, trolls, dragons, golems, other new mobs, underwater monsters, megabeasts
Trolls can throw rocks, goblins come in swarms
Monster "towns" can be found, either to play with or wipe out in infinite mode (e.g. goblin village, monster nest)
Ramp tiles (possibly to replace half-blocks)
Ragdolls for everything
Much more fleshed-out crafting tree
Animals can be moved/kept much easier, leashes/fences, etc, zoos possible
Maps are confirmed, possibly with custom annotations
Clocks and compasses confirmed, complete with animated textures
Crouching keeps you from falling off edges so it's perfect for building bridges
Diamond will be better, currently sucks
Ladders might be implemented if he can't come up with something better, main problem is FPS ladder controls suck (get stuck, fall by accident, etc)
Downloadable offline version coming when MP is released
Seasons: Changed rainfall, crop viability, ice and snow in winter, etc.
Multiplayer moved up in priority, seasons pushed back to after multiplayer
Smoke may be added, necessitating chimneys
Playing as a monster would easily be possible but boring without an inventory
Signposts are coming
Poll