The Left 4 Dead blog has some info on the "mutations" (ie game mode variants) that are being introduced with The Passing:
Our first Mutation is Realism Versus.
For this Mutation, Survivors have all the Realism changes working against them, no glows on items or survivors, tougher zombies, instant kill witches, etc. The Infected have all those changes working for them. This makes the Jockey one of the most powerful Specials for his ability to steer a Survivor out of view. You will want to make sure you have a working microphone so your teammates can hear your screams for help. This Mutation is only available in Versus.
Trailer posted below. Francis hates stuff, obv. Reckon I might be re-deading some zombies later...