From Grey in the Hawk
1) Player Resolution of Critical Events
The seminal events in Greyhawk's current history and development are all presented such that the players may not only take part but play a leading role. Players could fight the Greyhawk Wars.
Players defeated the hordes of the Temple of Elemental Evil. Players defeated Lolth. Players turned the tide as Iuz aced Vecna.
In the Forgotten Realms, for example, Ao decrees an event and the players get to clean up in the aftermath. Cyric destroys Zhentil Keep offstage and the players get to delve into the ruins. Gods die to be replaced by mortals and the players watch. Elminster sends players on a mission but ultimately keeps from them the greater goal the mission serves.
2) Persistent Personified Evil
Evil in Greyhawk is persistent. It is halted, checked or imprisoned but it is not defeated with finality for all time. The triumph over evil is a relative thing, ultimately transitory.
Evil in Greyhawk is personified. Evil has faces and names attached to it that ring down through the setting's history. It is not an evil that pops up purely to give the players something to strive against and defeat before moving on to the next evil that similarly appears out of relative nowhere.
Vecna, Iuz, Lolth, Tharzidun, the Scarlet Brotherhood, Aerdi, Kas, even Turrosh Mak, all met this criteria. They are highly personified forces that spring from the setting's specific history. By comparison, evil in the Forgotten Realms is of the pop-up variety save for the Red Wizards and Zhentrim. Menaces appear from nowhere or with on the spot histories that never before appeared in the setting. Greyhawk allows for this type of toaster villainy but it also established from the first, villains of a historic character that transcend the needs of the adventure of the moment.
3) Applied Internal Historic Consistency
Greyhawk has a strong internal sense of history that is consistently applied in all "Greyhawk" products or creations. However, not every product published under the name "Greyhawk" meets this criteria. Greyhawk is a storied realm. It's seminal figures, good and ill, are interwoven throughout the setting. It has a defined history that strongly influences the present and future of the setting. Greyhawk's history is not a footnote but an integral part of the setting that must be understood to truly comprehend the relationships among men, nations and even gods. True "Greyhawk" products or creations build on this history, incorporate it and develop it. The best such products or creations leave enough open ends to allow for further such development. More mediocre products attempt closure of every loose thread.