A significant new addition to the game is a new playable race, the "Lanius", metallic lifeforms that reduce oxygen levels in any room they are in.
Additional components added to the game include a clone bay that replaces the medical bay by creating clones of deceased crew with a small penalty against their skills, hacking drones that target specific systems on an enemy ship, a mind control system able to take control of an enemy crew member, a pulsar environmental hazard which periodically disable a ship's systems, and battery systems to give the player a short burst of power at their discretion. Other new features include a brand new ship and alien race, a third layout for eight of the now ten ships, new weapons, and many new beacon encounters, as well as a new sector. An additional Hard difficulty mode is now available as well. All of the expansion's content can be disabled within the game if preferred.
Source: Wikipedia, "FTL: Faster Than Light", available under the CC-BY-SA License.