It was (and is) a kitchen sink setting, with literally everything thrown into a blender together and then poured out. You want some Egypt? I got your Egypt. You want some techno-gnomes? We got that. You want traditional Western European fantasy? No problem. Some Asian style? Hey, we'll just excise Kara-Tur from Greyhawk and stick it in the Realms. How about some Mongol stuff? Sure, we'll add that! Oh, you want a New World discovery scenario?....
It's a matter of opinion, but to me, cramming every possible identity together leaves the Realms with very little identity of its own. The Mythals and dead magic and all that being some of the exceptions.
Greyhawk, on the other hand, has a distinct Dark Ages feel. Good is battered and under siege. Evil is on the rise. The big empire (the Great Kingdom) has fallen under the influence of demons. I have always felt that Greyhawk has a far more distinct identity, and one that is far more likely to inspire adventures for me. I get that a lot of people disagree, and that's okay, but that's my take.