The idea of a knight facing off against a dragon in Greyhawk is credible. In FR the same image is lah-di-dah; the knight is probably some magically empowered glowing dude, and dragons are just not as credible as opponents in a world full of the wild stuff going on in FR. Magic is a little harder to come by; not unreasonably, but it's clearly magic, and not corner-store enchantments and pseudo-technology. The tropes cleave a little closer to swords-and-sorcery. It replaces the dualism of LOTR or FR or Dragonlance with a moral realism that is embodied in specific people, creatures, and acts, not in monolithic armies. Civilization is very civilized, but also geographically isolated. It's a little easier to believe in peasants carrying out their lives in Greyhawk than in other settings. Aesthetically, it resembles the middle, middle ages more, and an airbrush painting on a van a little less.
Probably the most striking thing about it was that it was born out of the tropes of AD&D and then fed back into them, but itself never seemed to be constrained by them. It introduced antipaladins, for instance, and gave us our first taste of the Drow. The elven races paralleled the PHB ones but were not precisely them. Iuz was a half-fiend, something not even statted in AD&D until he came along. It was very much a version of Gary's homebrew.