Aside from the obvious differences (geography, famous NPCs, pantheons, etc.), what distinguishes a real Greyhawk campaign from a Realms campaign or a campaign in another setting? How do you define Greyhawk essence or spirit for yourself? Nowadays, obviously, there's a strong emphasis on making adventures portable, so they're useful to the largest number of gamers. What would you add to a generic adventure to make it really Greyhawk?
Any thoughts welcome and appreciated.