It would appear that the engineers at Valve (and formerly of Turtle Rock Studios) have spent a considerable amount of time making Left 4 Dead look, well, ever so slightly different. Side-by-side, with all of the game's visual tricks switched on and off, it's clear they've gone the extra mile to ensure that the zombie shooter looks pretty in between those brief moments of respite from the hordes of undead sprinting at you.
This is the kind of minutiae that will likely get PC enthusiasts turgid and interesting insight into the stuff that might pass you by otherwise. Shame that when I play it on my own Windows box, it'll be dripping with "effects off" visuals.
L4D Art Direction, Part 1: Filmic Effects [Left 4 Dead Blog]