• In the Fallout 3 universe, Fallout: Brotherhood of Steel and Fallout: Tactics never happened.
• Fallout 3 will feature a day/night cycle as well as changing weather.
• There will be no multiplayer mode. No demo is currently planned.
• Players can collect 21 collectible bobbleheads throughout the game
• Fallout 3 will feature 20 licensed songs from the '40s. Players can listen to these songs through radio stations accessible via the PIPBoy as well as radios that players can come across in the game world.
• Travel between locations can be done via subway tunnels. There will be no drivable vehicles.
• Bethesda is still considering the addition of downloadable content as well as player-created mods.
• The game will have 9 to 12 different endings.
• Almost all NPCs in the game will be killable. There will also be children, but Bethesda has yet to decide if they will also be killable as in the previous Fallout games.
• Players will once again be able to hire mercenaries for extra firepower. As in Fallout, players will not have a lot of direct control over them.
  

Via Joystiq