What's so good about Greyhawk?

The most important thing about Greyhawk is it's a Roleplaying world and campaign setting first. Other popular campaign settings are Novel settings first (FR, DL, etc) and campaign settings second. When I ask someone a regional question about Faerun they'll tell me what Drizzt or Elminster did. The same with DR (except it's Soth or Raistlin, and so on). If I ask someone about anything in Greyhawk they'll be able to tell me the general history, then tell me about their adventures.

The shared experience in Greyhawk is the modules (Against the Giants, Descend into the Depths of the Earth, Slavers, Tomb of Horrors, White Plume Mountain, Temple of Elemental Evil, etc, etc, etc...). In FR and DR the shared experience is the novel. Their characters in both worlds are shadows of the Mary-Sue, the DMPCs. As a result, look at the core settings for each and you'll find the simple fact - if you ain't epic, you ain't $#!+. In GH there are no hard-coded events, not at the same level. The history of GH is peppered with events that were altered "by the deeds of brave adventurers." Those words are found throughout the setting. The same cannot be said for even Eberron, which is largely a Roleplaying Setting first.

Greyhawk is staple fantasy but far from generic fantasy.

Greyhawk is full of detail but open to interpretation.

Greyhawk has wonderful NPCs but they need your help.

Greyhawk is D&D.