Dark and Grim are two separate things, and WotC will go dark, but not grim.
Ask any GH fan what makes their setting different than Forgotten Realms. You'll get a lot of answers ranging from nostalgic to smug, but the big one seems to be Greyhawk is grim. Races are very segregated, nations are isolationist. There are plenty of "monsters" but "PC races" are few. Groups like the Scarlet Brotherhood are forged around racial identity and purity, slavery is accepted in several areas, and every statement about orcs, drow, and the like are true. It flies in the face of the current trend towards more inclusive settings, less overt fantasy racism, and the notion of "play what you want" that is immensely popular with newer players. You could make Greyhawk start to resemble that kind of setting, but you run into the 3e problem of "It's like the Realms, only less well known".
FWIW: I think their Dark Sun will continue down the trend of 4e's take on the setting, with maybe even more concessions. The core of the setting (brutal survival, psionics, desert wastes, distant gods, corrupted magic) all work simply fine even if you're a tiefling bard. They can lighten up Athas and keep the flavor correct, much like how they lightened up Ravenloft and kept it on flavor.