Campaign settings that took D&D in different directions (Greyhawk, Realms, Al'Qadim, Mazteca, Kara-Tur, Birthright, Dark Sun, Planescape) instead of just further down the road in one direction (High Magic, Super Magic, Cheesy Magic... Oh, 'scuze me... I meant "Greyhawk, Realms, Ebberon", really...