I used to be a pretty serious D&D player, and for the most part, I was glued to the Greyhawk setting as the backdrop for my games. In one of my longer bouts of ambition, I put together a map of the Greyhawk Cosmology. This was as comprehensive a list as I could develop at that time, given the resources I had, of the gods and other deities and planar beings of power in the world of Greyhawk.
It was a rich set of sources to draw from. The back-stories for malevolent evils such as Tharizdun were perfect for antagonist cults and the like. In fact, everything about the cosmology was ripe for story-telling. The only thing that I was frustrated by was the lack of new source material, especially when Wizards of the Coast shut down Paizo's ability to publish Greyhawk-based Adventure Paths.
Today I look back and wonder if I'll ever have the ambition to put a Greyhawk game together again. It's so much work, but I love the setting and every time I've worked with it before it's been worth the trouble.