Nostalgia, big time.
The first product I bought for AD&D2e, rulebooks aside, was The City of Greyhawk boxed set - before that I only played BECMi, and had a tresure trove of Mystaran Gazeteers.
Stuff already said: big picture with details, plausible history and people, nations that don't look like "a touch of this a touch of that", a feeling of danger pretty much everywhere without the need of leveled farmers and bartenders.
To me another big point was the proactiveness of NPCs, those high level legends that are DM tools rather than statblocs. Bad guys didn't just brood in dark and foreboding keeps 'cause they were eeeeeviiiil, and good guys didn't just wait in line to bash evildoers when they showed up.
Just reading the setting you had the feeling you had to keep up with the pace, there was a lot actually in motion and not in the "just about to happen" limbo.
The only setting that managed to come close was the Scarred Lands during the 3.5 days.