If you beat the game on Legendary mode, there's an extended ending where after the Chief escapes in the ship, you see Sgt. Johnson and an Elite fighting each other a small distance away from the crashed Pillar of Autumn. The Sergeant and alien notice the ship's self destruction, knowing it's their last moment alive, they stop fighting and hug each other.
The Xbox version had cooperative play and multiplayer limited to split-screen and system-link. The PC version did not have cooperative play, but it had online multiplayer instead.
There is a flamethrower in the PC version but not in the X-Box version.