That's cute, but you're not disagreeing with me either. It is obviously hard to tell what's unique about the setting.
It's not that it's unique, per se, it's that it was the first.
The various settings it seems like? They were ripping off it, not the other way around.
Greyhawk suffers from the fact that it's been poorly marketed for 30+ years. It was always so intrinsically tied to Gary Gygax that once he was out at TSR, Greyhawk was never a priority for them, and it showed.
90's Planescape books strongly hinted that Oerth itself was dying and might cease to exist soon, TSR in that era saw it as a burden and was vaguely hinting at some looming world-shaking event that might outright destroy the world. What few Greyhawk works there were in the 90's weren't widely distributed and weren't well promoted. . .as a new D&D fan in that era, Greyhawk wasn't even on the radar of settings to study, everyone was into the Realms (or maybe Planescape or Ravenloft).
The Realms became popular because of novels, and tie-in video games, and so many similar other things beyond traditional D&D books and modules that told the story, so people who were only tangentally part of D&D culture could be brought closer to it, and that helped grow the Realms a lot. Dragonlance got big like that in the 90's too (before Dragons of Summer Flame chased so many fans away by essentially ending the setting, the whole "5th Age" stuff never caught on).
3e made an attempt at a limited revival by making Greyhawk the presumed default setting (which it always was to a lesser extent, like was shown in spell names, but was treated a little more obviously in the 3e core books, like listing Greyhawk deities as player options to choose as a patron deity).
Greyhawk isn't an influential or major setting because it's unique, it's because it's the seed that so many other things in D&D sprouted out of, that it has a special and unique place in D&D history and lore.
