Spinning off a bit from some discussion in the Dragonlance thread.
D&D derives a lot of its aesthetic and assumed setting from the medieval to renaissance period in Europe. I've just been thinking about some of the historical factors that were enormously prominent, everyday, and important in the real world, but which D&D either neglects completely, or just casually makes assumptions about without really thinking about how the everyday presence of magic and the verifiable existence of a polytheistic pantheon of gods would affect it.
Tax, for instance. It was probably the cause of the majority of medieval wars and uprisings that weren't caused by religious differences. When was the last time your PCs paid taxes? Who does pay the tax in your campaign world? How does money get raised to build castles, city walls, sewers, and roads, or to pay soldiers and bureaucrats? Higher-level PCs are some of the richest people in a campaign setting (other than dragons...) - if they're not paying tax, who is?
Monarchy. Historically, monarchies almost always drew their legitimacy from the endorsement of the Church, who spoke for god. Divine Right of Kings and all that. Are your D&D campaign world monarchies divinely endorsed? If so, by which gods? What do the residents of the kingdom who worship other gods think of this? Does the endorsing god ever revoke their endorsement when a monarch goes off the rails, or starts worshipping something else, and if so, what happens then? If the monarchy is not tied to religion, where does it draw its legitimacy from? And the same with all the lesser aristocracy further down the chain.
Land. Who owns it? Who decides who owns it? There's a lot of Generic Wilderness in most D&D settings, in your setting is this on nominally owned by someone and it's just too monster-haunted for them to use, or is it legitimately unclaimed? Do you have a
squattocracy of powerful adventurers picking bits of land out that they like, slaying the monstrous inhabitants and setting up domains of their own regardless of who owns the title in a dusty ledger hundreds of miles away? And even in more rural and tamed areas, or in cities - who owns the land? Is your average farmer or shopkeeper a yeoman who owns a small holding of their own, or are they tenants paying rent to a noble or landlord? And in a world of dungeons and underdarks and subterranean dwarven cities how
deep does title run?
Spell lists. Most D&D spells are intended for adventurers obviously, because that's what the game is about. But that leaves a lot of conceptual space for spells that'd be incredibly important in the non-adventuring world. The Ceremony spell is a nice gesture in this direction, and Plant Growth has some great agricultural utility, but there's some fairly obvious gaps. Does your world have people researching/casting spells like Ease Childbirth, or Improve Dwelling, or Increase Fertility, or Accelerate Fermentation, or Grind Grain, or Contraception, or Permanent Dye? If not, why not? Are there any other obvious utility spells that a pre-modern society would have developed that DON'T centre around bashing monsters?
Punishment. Pre-modern societies were very big on corporal punishment. Floggings, hard labor, and mutilations and so on. There's some fairly obvious Rule Zero reasons that this stuff wouldn't be welcome at a lot of tables, but in that case, how do minor crimes get punished? Corporal punishment works because people are afraid of pain and injury, but this is D&D, when most injuries disappear after a good night's sleep and pain (or deprivation, or discomfort, or boredom, or bad food) only matters if you choose to roleplay it. What sort of punishments do your rulers apply? Are there magical options - application of spells like geas?
Social mobility. Ancestry and birth were historically very important. Even if you were rich and successful, if you were low-born you could almost never truly achieve equality of social status with those who were born noble, or high-caste, or royal. How tightly do the nobility guard their class integrity in your world? Can your successful adventurers purchase a title, or can their wealth open doors into high society and will they be genuinely accepted there? Can the commoner hero marry the princess without comment, revolution, or ostracism? But if there's no meaningful dividing lines between noble and commoner, what does being a noble even mean?