Please note, that my statements assume the original Greyhawk Folio, the Greyhawk Boxed Set, and Greyhawk adventures printed through 1984. That is Gygaxian Greyhawk, and nothing beyond that counts for me (even if I steal good ideas from it occasionally).
Greyhawk is the original official setting, designed by the grandfather of D&D, Gary Gygax himself. While I'm quite certain that won't mean much to many players, it is important to note, since many tropes of the game came about from this campaign setting (such as most named spells, like "Tenser's Floating Disk"). It has a rich history, but much has been left undefined, allowing the DM freedom to customize the setting without violating cannon (which FR and Dragonlance often struggles with). The world isn't a massive struggle between good and evil, but rather a realm where alignment dictates politics rather than allies (two of the most LG nations dislike and distrust each other). It's very sword and sorcery, in the vein of Conan, Elric, and Fafhrd & the Gray Mouser, which is dark and gritty with evil magic and decadent societies. There are very few powerful NPCs and few orders of "good guys" there to save the day. It was the original Points of Light campaign, with danger at every turn and vast riches to be found.
Later Greyhawk works by TSR and WotC attempted to bring it more in line with the FR style, which watered down the setting and drained its magic away.