Left4Dead2 is the shit, its not revolutionary like L4D was (obviously) but they really managed to turn it into L4D on steroids. Which I totally like. Plus nothing beats slashing zombies five at a time with a huge sword....
With that said, obviously this kind of gameplay is "different" enough from regular shooters that it wont appeal to everybody.
Id like to see it in some kind of Sci-Fi setting, and if Valve doesnt do it Im pretty sure someone else will.
I have a nifty blue line!