L4D was missing the horror in survival horror. I think many people, including me, were disappointed because L4D lacked the features that most people expected from a co-op zombie agme. L4D had infinite ammo, third person view, limited guns, and melee spam. Sure you could shoot limbs off but the gore seemed lacking. I would have preferred to see Zoeys head punched off by tanks and Bill's intestines ripped out by a hunter.
I think L4D was too streamlined and casual to be horror. It felt like they removed all of the "hardcore" features in order to appeal to a wider audience. Essentialy, they made it too simple and took out a lot of the depth that you would expect from a zombie game.