Valve has been candid in the past that one of the secrets to its game design success is to test, retest, and then test some more. So it shouldn't surprise you that the company's iterative design approach extends to all the minutiae of game production, even something as seemingly insignificant as the box cover. Valve artist Jeremy Bennett weighs in over at the official Left 4 Dead Blog on the process of getting the cover art just right--which, in this case, appears to have meant cycling through several somewhat lousy examples before completely switching gears and coming up with something amazing.
Magnifying the problem was the fact that there wasn't much point to having our four heroes standing around in a void. To give the box art any sort of context to the game inside, we needed zombies. So we were faced with the unpopular idea of shrinking our heroes down even more to cram some zombies into the picture.These attempts did a good job of capturing the multiplayer aspect of L4D, not to mention the intensity of the gameplay experience. But when viewed from a distance—by someone walking through a game store and spotting it on a shelf, for instance—it just looked like cluttered noise. The individual elements were too small and varied to communicate anything visually interesting.
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As Mr. Bennett mentions, the one and only real purpose of a box cover is to get people to notice and consequently buy your game, but at least you can make it look cool in the process.