Novelty
Any setting I ever liked had lots of novelty - new races, classes, themes, forms of magic, etc... Any setting has to have engaging novelty, something that you'd love to get out and roleplay, experience in game, and read up on.
The bad thing about novelty for me is that once you've experienced it, seeing it appear in another setting just doesn't thrill as much.
Here are a few TSR/WotC settings and the new things they had which made "blipped big" on my radar" or were novel but not thrilling.
Eberron - WOW: Warforged, Artificers, Magewrights, lots of low level NPCs, a world weary war finding its way, the Houses, an arcanely infused world; MEH: the way Eberron does drow, dragonmarks (too much like Birthright's blooded abilities), the lightning train (a little too Polar Express)
Forgotten Realms - WOW: rich variety of high-level magic, cool countries, the presence of so many fascinating religions/gods interwoven in everyone's life, the Time of Troubles, loads of epic storylines in novels, memorable NPCs; MEH: the Sea of Fallen Stars and other underwater books, the real world additions like Maztica, the Alaska-like arctic sourcebook with eskimo-like language which seemed like an issue of National Geographic with the names replaced.
Dark Sun - WOW: Psionics for everyone; savage world fallen from grace, elf-eating thri-kreen PCs; elf-eating savage halflings; advanced beings, the Valley of Fire & Dust, lo-tech, no metal, outlawed arcane magic, no water; MEH: The second edition changed the game in scope with lifeshaped things, lush lands outside the boxset that betrayed what the setting was supposed to be all about, thri-kreen that came in a variety of colors, shapes, and purposes.
Birthright: WOW: blooded individuals, that age-old god-war, the unique monster characters, the war game, domain turns, blood silver, tons of interesting nations statted out with maps; MEH: no meh here
Planescape: Planetouched, Factions, faction politics, Sigil, opening up the planes for all level PCs, cant, a place where philosophy ruled; MEH: Sometimes too mysterious for its own good, numerous background story arcs that were hinted at, advanced slowly, or rarely followed up on (although I'd be hard-pressed to list them at the moment. The arcs could also be considered open-ended setting material for DMs to use as they see fit but I wanted to see them spelled out!
