Judging from the feedback I read, the pro's who were invited ended up wasdaing around with p90's because the rifles sucked for headshots, or P90 was just too easy and powerful
Valve are selling it as a console game, so if they want success there they have to gear the gameplay for the control method. With a gamepad you won't see much recoil and headshots will have a reduced importance. They will also have to keep in mind the younger (on average) player age, and expectations from console players about FPS's
They are updating Source anyway, and the GO graphics are built off L4D 2 engine so won't be much different. If they work on Source they can bring the 1.6 and Source community together better than a whole new game with dumbed down gameplay