From Joystiq, what we know so far:
# The game is set 3,600 years prior to the events of the Star Wars movies, and 300 years after the events of Star Wars: Knights of the Old Republic II.
# The plot centers on an impending war between the Sith Empire and the Galactic Republic.
# BioWare claims the game contains enough dialog, plot, quests, and other content to comprise KOTOR 3, 4, and 5. It is larger than all previous BioWare games combined, according to the developer.
# There will be multiple classes for both the Sith and Republic factions, each with their own storylines that will carry players from the first level to the last. No dialog, quests, or content will be reused between classes.
# There will be no cross-over between the two factions.
# Jedi and Sith are the only classes revealed thus far; lead writer, Daniel Erickson, "loves Mandalorians."
# You will be able to interact and upgrade multiple companion characters, but only one can accompany you at a time.
# There are no "kill 10 rats" type quests. The most basic creature you will battle as a Sith at level one is a 20-foot-tall K'lor Slug.
# Space flight will factor into the game (for traveling from planet to planet), but BioWare isn't saying if it will include space combat.
# There will be a crafting system. No further details, other than implimenting it was "a toughie," according to the game's lead writer.
# The Old Republic will use a level system, but the level cap "can't be shared just yet."
# Combat will be faster-paced and "more epic" than in competing MMOs to "capture the Star Wars feel."
# Characters from the Star Wars expanded universe will cameo, as will ones from the KOTOR games.
# The designers have created a new mechanic in order to encourage players to choose classes other than Jedi and Sith, but it can't yet be talked about.
# There will be PvP in the game; it will factor in the Sith and Republic factions and "feel important in that context."
# BioWare intends to include a mentoring system allowing players of varying levels to adventure together, but hasn't decided if it will take the form of a sidekick or a downsizing system.
# Vehicles are "on the plate," but can't be talked about at this point.
# The game will include "a form of instancing," with the design team saying that, "being able to do storytelling, being able to do experiences that aren't disrupted in public areas," is a high priority.
# Finally, BioWare is only talking about the PC version at present, but is considering console versions.