Dire Bare
Legend
The penny dropped for me on what keeps AD&D 1E and Greyhawk in particular a bit different from generic fantasy setting #19902 (e.g. Mystara, Thunder Rift, Dragonlance, Birthright etc).
It's playing up the sense of the eldritch.
For me, it's not so much a sense of the "eldritch" as a sense of mystery. When I was a young DM collecting the D&D Gazetteers (Mystara), each one provided a wealth of detail yet left tons of mystery about the world. My favorite bit was the hints of the existence of the Hollow World in the Thyatis/Alphatia boxed set. This didn't last, of course, as the setting became more and more detailed.
Even when Eberron was first introduced, I didn't get that sense of mystery. The setting seemed very "firm" and detailed from the get-go.
The reimagined core setting of 4e gets back to this somewhat, but since it is based on classic D&D each bit of revelation is more of "how things have been reimagined" rather than a revelation of a new bit of lore.
I'm looking forward to whatever the next brand-new setting from WotC will be, and I hope that the two-book, 1 adventure, and occasional support articles in Dragon/Dungeon will bring back the sense of mystery for me!