Attention to detail is what made Left 4 Dead special. You can tell there was a lot of thought and passion that went into creating the game, and there wasn't a bunch of pencil pushers behind the scenes making decisions on the game's direction.
@HBM: According to Digital Foundry it's because of how the artists layered the large variety of art assets. Their goal was to create a cohesive image at all angles which involved a lot of layers of art assets, but because of this usage of resources they had to compromise a bit on texture quality.
@doctorj11111 Other games have been released since then such as Path of Exile, which has also been constantly improving. So Diablo 3 has to compare to those other games at this time.
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