Since I just happened to post a reply with some of this information the a different thread, I'll repost it here:
Since I run a pretty much standard Greyhawk game, I understand that there are European style knights that come from the Shieldlands, the Northern Kingdom, Furyondy, etc. There are Arabic style people's who come from the west, from Ket, Zeif, Tusmit, etc. There are powerful mage guilds in cities always discovering new magical abilities and styles. There are people who are nomadic and shamanistic leaving room for other styles.
I so far have only disalllowed things purely oriental in nature as there isn't a major oriental culture in the Flanaess. There MIGHT be one elsewhere in the world, but most of the area outside of the Flanaess is not really explored or canon. Which means I can use all of those PrC if I want as one time enemies or as storyline devices.
As for almost all the creatures in the MM, MM2, FF, and pretty much every other book, they've been proven to exist somewhere and there are areas that have been unexplored where the rest could come from.
So, overall, I find that it is a good world because all of the material put out can fit into it without straining belief. This is actually why I chose Greyhawk as a world. It allows me as a DM to come up with almost anything without restricting myself to what may or may not exist in the world. If I feel like a high seas adventure, I can run one, if I plan on sending them to a desert and using Arabic enemies for a week, I can. A lot of my fun comes from seeing the PCs deal with situations that put them out of their element. They don't know what the enemies can do to them, because the enemies could be ANYTHING.
As a secondary benefit, it gives me excuses to allow almost anything as a PC as well. Although I restrict races to those in the PHB and subraces of those races. Also, each final character has to be run past me because although individual classes or feats are not overly powerful, certain combinations are. I've rejected only a couple of ideas though.
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As a bit more directed answer, I see Greyhawk as sort of like Earth, there is a mix of countries, cultures, personalities. They can be at war, they can have alliances. There is the unknown. Some people have never ventured deep into certain valleys and forests or beyond the main continent.
Then there is evil. There is a powerful demon, Iuz who actually controls an entire country. He is too powerful for anyone to completely oppose him though, although they try. Other evils seduce people all over the place though, trying to establish a foothold in the world.
There are ancient empires, powerful magics forgotten, waiting to be recovered by adventures.
I like that Greyhawk can be whatever you want.
If you're wanting general info, why such specific queries? 
