1: Nostalgia. I have to admit that growing up with this setting when I was a teenager does give it a special place in my heart.
2: Fuedal European feel. More than any other setting out there, Greyhawk captures that. You have dozens of sovereign states, each one with a good overview - resources, boundaries, leader, etc, etc. This is completely lost in Forgotten Realms - you have a few city states and evil bad guy states, but that is about it. This aspect really made the world come alive for me.
3: Mystery: Take Forgotten Realms. EVERYTHING is listed in minute details in hundreds of supplements. The world is over saturated with magic - hell, WotC officially condoned players being able to buy any magic item they wanted in the Magic Item Compendium. In Greyhawk, magic is rare enough to have mystique about it. The way the books were set up, you were given a nice framework and the DM filled in all the spots in between. It made the world unexplored, it gave you a true feeling of the unknown as you traveled to a far off land.
4: $$$ - or the fact you don't need to spend a ton of it on Greyhawk. A couple old 1st edition supplements (the Gygax stuff!) and off you go. Gary designed the world to be a fun place to play, not a way to sell book after book. Not only is it cheap to get the material, but the lack of it keeps your players from getting angry when you deviate from "official" material, because the books don't cover the entire planet in microscopic detail.
After 28 years of gaming, I still love Greyhawk more than any other fantasy world. I still feel like there is a huge part of the world I have not yet touched with my campaigns, and I hope to never see its grandeur fade.
Bingo! Good points and an excellent post. You want to go to China like some of GG's campaigns did from GH, go ahead. Want to add the Wilderlands to the other side of the globe? You got it. The Flanaess is big enough for most campaigns and the uncharted territories outside of it is the DM's playground.
AD&D, Amish Dungeons & Dragons.
"Galstaff, ye are in a cornfield, when a moustachioed man approaches. What say ye?"
"I shun him."
"Knowledge, logic, reason, and common sense serve better than a dozen rule books."
-- E. Gary Gygax