There is a realistic game mode. In it, there are no revives, the weight of the gun/ammo will slow the player down, there is very limited HUD, heals take longer, and require the medic to dress to the wound, not hit markers and it's almost always a one-hit kill when shot.
The ability to customize your soldier's nationality (ie Black, White, Asian).
The PLR and MEC in multiplayer
The Panama Canal in a future map pack
A leadership system where when players win/get leadership points, it goes towards a score for them which helps determine who the squad leader is.
Although I don't play on the PC, mod tools for the PC.