Greyhawk is, for us, generic D&D fantasy. This isn't surprising. The Forgotten Realms likewise feel like generic D&D fantasy these days, but Greyhawk has some significant differences that make it a lot more fun for me - at least. (I love the Realms; I love Greyhawk more).
Most important in that is that it is a land of nations. If you think of the main area of the Forgotten Realms - the Sword Coast - which we've been dealing with for the past 10 years, can you name a single nation in it? It has city states. There are nations in the Forgotten Realms, but they're all off the edges of the current map. Even when you get to the lands of Cormyr and Sembia, in which I spent many a happy day in the early days of the Realms, they're surrounded by more city states and wilderness.
But you look at Greyhawk, and the map is full of nations. And they don't get along. Once upon a time, most of the map was ruled by the Great Kingdom, but it's been falling apart for centuries now, and it's corrupt and much smaller. The new nations are struggling along, making alliances and engaging in petty wars - but there are darker threats, such as the Land of Iuz, the evil cambion ruler of a land of orcs and mercenaries, all ready for conquest.
Iuz had been gone for a century - trapped by the Mad Archmage beneath Castle Zagyg - but he then returned after being freed by a foolish adventurer, and now he hungers for conquest once more. Of course, his land is no longer united - one faction, the Horned Society declared independence and now holds his eastern capital. He has to get it back! But once he does so, he'll be ready to take on the southern lands once more.
The lands immediately to the south are Veluna and Furyondy. Veluna is a theocracy, Furyondy is a kingdom. They are kingdoms of good and wise rulers. However, a few years before the present in the original boxed set, there was an uprising of worshippers of Elemental Evil. Both sides raised armies, and they clashed at the Battle of Emridy Meadows. The armies of good were victorious, and the Marshall of Furyondy - Prince Thrommel - was the victorious leader. His hand was pledged to the daughter of the ruler of Veluna, and by such would the two nations be united. They might even be able to take out Iuz!
But such didn't happen. One night, the Prince vanished, and the plans for unity were dashed. No war started between Veluna and Furyondy (though it'd be in keeping with the setting), but the Prince is still missing as of 576 CY. In fact, he was taken captive by followers of Elemental Evil, and imprisoned beneath the ruins of the temple - players can find him there. In Monte Cook's follow-up (591 CY), he's been turned into a vampire, as he wasn't rescued. (In my campaign, in 630 CY, his artifact sword, Fragarach, was recovered and returned to the new young and foolish king of Furyondy. He declared war on Iuz, seeing it as a sign. The war did not go well!)
The nations of Greyhawk are distinct. You can imbue them with personalities. It comes from the wargaming background of Gygax, and though I don't engage in resolving wars through miniature wargames, they have taken place. (The disappearance of the High Priestess of Xan Yae on a visit to Veluna caused a war between her homeland of Ket and Veluna which the players had to stop. Well, by winning it for Veluna).
There are the ruins of ancient kingdoms. There are the effects of the migrations after the Twin Cataclysms when two empires destroyed each other. There are the rise of the Younger Kingdoms, and their fall and rise again. There is wilderness, there is evil, there is good. And some of the most famous dungeons in D&D history.
Cheers,
Merric