One that feels genuine. I got tired decades ago of the JRRT-standard Shining City In A Sea Of Empty that seemed so beloved of setting designers: you just cannot have glittering civilizations that give way to hostile wilderlands and howling orc hordes a bow shot from the capital's walls. I don't want a terrifying war fleet without the maritime infrastructure to support it, or jackbooted legions without the secure farmlands to feed them, or hundred thousand person cities in the middle of a desert with no particular sources of food, water or trade. I don’t need Good! Vs! Evil! to be the motivating factor behind every international, intraurban or internecine dispute; good old human motivations will do nicely.
One that isn't set up as a Potemkin front for the care and feeding of PCs. If there are five wizards in town, they're not sitting around twiddling their thumbs waiting for the PCs to put in enchantment orders. I'm happy to see one working for the Baroness, one working for the town's richest merchant, one playing communications relay for the national military, and one being a drunkard no one with alternatives trusts any more. (The fifth one can fit them into her schedule, sure, if they pay her enough to bump the waiting list down a bit. They're cool with her being an earth specialist whose work is mostly in creating and shaping stone, right?) If it's a 500-person town, there isn't unlimited gold available to buy the party's loot, and the townsfolk are only interested in buying things they can actually use. Why would the local knight want to empty his treasury to buy that magic sword? He's got a perfectly good one already, and he's got bills to pay, retainers to feed and the annual taxes to cough up.
One that isn't set up as a Potemkin front to oppose the PCs: The party was trying to break into this three-story building that housed a jeweler. Not someone who stashed the Crown Jewels, not a great enchanter, just a small-town jeweler. And there were no windows or chimneys in the building. No magics for light or air circulation. No explanation as to where the smoke from lamps went. No windows-but-heavily-barred. No rhyme or reason, no sense to it. No rationale beyond "haha, this building is designed to thwart PCs and force them to do a frontal assault during business hours." (You may safely conclude that I was done with the campaign after that session.)
One with detail work in the right places. I don't need the full gamut of combat stats and skills for the twenty lead figures in the royal court -- what are the party's chances of dueling the palace chamberlain, unless she’s really the Big Bad? I don’t need a trap on every chest and every cabinet in every room of the palace -- good grief, the chambermaids and scullions will all be wiped out by Saturday -- and I don’t need detailed maps of the dwellings of the blacksmith and the chandler and the apothecary. Put that work into the NPCs (and the items) with whom the party interacts at that level.
One where “it’s magic” isn’t the blanket excuse for everything. You won’t let me snuff the Big Bad or whack out her minions with no more effort than a snap of my fingers just because I say “I’m the hero,” would you?