After reading this thread, and trying to get a feel for Greyhawk from the Greyhawk partisans, I have the following related thoughts:
I feel like there are two types of D&D settings:
- The adventuring party reflects the setting
- The adventuring party is significantly different than the setting
Forgotten Realms and Eberron are Type 1. Ravenloft and Dragonlance (in the War of the Lance, at least, due to clerics) are examples of Type 2.
It feels to me that Greyhawk is a Type 2 setting, and that's the main difference between it and the Forgotten Realms.
Specifically, as everyone says, Greyhawk is a Swords & Sorcery setting. To me, the difference between S&S and other Fantasy is that S&S does not have "magic". It has "sorcery".
Sorcery has very different connotations than magic. Sorcery is tainted, unclean, malevolent, corrupted. It's the difference between
saidin and
saidar in Jordan's Wheel of Time. At best, major NPC sorcerers like the Circle of Eight are neutral, rather than good. Most end up evil, though.
Magic, on the other hand, is a neutral force. It can be used for good, or it can be used for evil.
The thing is, though, D&D PC adventurers have "magic", not sorcery. That's perhaps a subtle difference, but I think it's hugely important, and is ultimately the source of the disconnect. The way the PCs interact with magic is very different from the way the rest of the setting interacts with magic, and there is no explicit explanation as to why. It's all a matter of tone, and what looks like arbitrary writer's fiat.