Realism Versus mode will be returning to Vavle's Left 4 Dead 2 permanently, Valve has announced, following an official vote.
Mutations are multiplayer modes that change on a weekly basis, and Realism Versus was the very first one. Clearly it proved to be popular enough for Valve to take a vote asking players if they'd like it to return permanently, 68% of which voted 'yes'.
'Realism' in Left 4 Dead 2 basically takes away all HUD assistance from players; there's no blue outline around team mates so you have to use communication to know where each of you are. Also, zombies are tougher to kill, with shots to the torso doing little damage making headshots far more essential than in non-Realism modes.
As you can imagine, it's incredibly tough, but L4D is known to fans for its break-neck difficulty. They're gluttons for punishment.