For us Left 4 Dead 2 its many more than normal Versus,Campaign,Scavenge or Survival

What is L4D?
Left 4 Dead and Left 4 Dead 2 is a co-op action horror game that casts up to four players in an epic struggle for survival against swarming zombie hordes and terrifying mutant monsters. Created by Valve, creators of Counter-Strike, Team Fortress, and Half-Life.

What is Mutation?
Mutations is the name for a set of gameplay modes that were released alongside the campaign, The Passing in Left 4 Dead 2. The Mutations are special game modes that run for one week before it is retired, and replaced by a new Mutation each week as they become announced on the in-game Blog Post every Friday.

"Each week one new Mutation is featured and the previous Mutation retired."
―Valve


You dont want wait for other mutation game?
This group is ideal for you!
We dont wait for next mutation!
We just play!
(By the way we welcome here also all players from L4D1)

To list of all official mutations here:
http://left4dead.wikia.com/wiki/Mutations