It's not so much that Greyhawk is "generic". D&D is Greyhawk, and Greyhawk is D&D. In AD&D1, about 75% of everything published for AD&D was part of Greyhawk. All the wizard names in the PHB were GH magic-users. All the named magic items and artifacts and relics in the DMG were GH items. The MM, FF, and MMII were the monsters of GH. Most of the adventure modules published for AD&D were placed in GH. In essence, the whole of the game was resource material for GH.
With D&D3, the designers went back (after the AD&D2 departure) to making core/default D&D, Greyhawk. The gods, the wizards, the magic items, etc. were all again acknowledged as GH.
It is not that GH is generic, or bland, it is that GH is the core/default/baseline for D&D. All other settings are identified by how they differ from this baseline. When another setting introduces a new "item" (race, group, class, magic, etc.), the designers make new rules to govern it. So that new item becomes unique and stands out as an example of that setting. With GH, everything already fits the core rules because, well, the whole game was designed for GH.
Sort of like saying there's nothing interesting about Earth when compared to all the strangeness found on the other planets and moons in the Solar System.
Quasqueton