L4D2 will feel and Run exactly the same as L4D1, its just gonna have new weapons/characters sounds and maps. They are doing sod all to the source engine.
ON GTTV Gabe said, "only reason why we are creating L4D2, is because we have so many great ideas, the current source engine cant handle"
I.e on the new L4D you can break limbs for the body and but bullet holes in them
