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.
I'm not sure D&D "neglects" those effects as much as leaving it up to the creator of campaign worlds to decide. Any speculative fiction on this topic is always just going to be guesswork and there are many, many ways it could affect things.
For example, magic works. Check. Are the gods actively involved or distant? How many priests are actually granted clerical abilities? How does anyone really know whether or not deities are "real" and that clerics are not simply manipulating magic in a slightly different form from wizards? In a world where powerful illusions exist, could mass believe create even more powerful illusions? Conversely if the gods are real and truly created the world and are actively involved, what kind of impact is
that going to have? How much free will do the different races really have and how much is the influence of gods?
There are a lot of different options.
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.
I don't bother with taxes any more than I make people track how many arrows they have. It's assumed to be part of their living expenses.
In my campaign world, some areas are monarchies, some are democracies, some are a hybrid with a council and a monarchy. The church tends to be advisors and, while important, are not the monolithic central power that the church was in Europe. In part that's because it's a polytheistic society, in part because there are different religious beliefs. So it just depends on where you're talking about. In the area I primarily run campaigns religion is important but it's just sort of integrated into everyday life, you don't go to church on a holy day but you will likely do small rituals and remembrances here and there. There are temples, but they tend to not be dedicated to specific gods.
In another region, ancestor and spirit worship is the norm. There is an emperor who is said to be advised by all of the ancestral emperors and is almost regarded as a holy being.
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?
There is a lot of land that is not controlled by anyone, especially in the current timeline because over a century ago 2/3 of the population was wiped out. It's a time of rebuilding and reclaiming, but honestly I rarely focus any campaigns on conquering the wilderness or similar. There is an ongoing fight against (literal) extinction, especially now that the "civilized" races took such a major hit, but colonialist-adjacent stories have never been my style.
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?
I assume there is a lot of low level magic in the world that make peoples lives better and more efficient. Magical cures especially greatly decreases childhood mortality, and there is corresponding birth control herbs. This means that it's less of a patriarchal society in most cases because women do not need to be constantly having children in order to maintain population levels. Magic helps with crops and livestock making farms nearly as productive as modern society. Particularly in long lived races where decades or even centuries may go between births, the balance between the sexes is different.
I don't have a completely magitech world like Eberron but many people use a little bit of magic throughout the day without even realizing.
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?
This is something I discuss in session 0. Punishment is harsh and often brutal by today's standards. There are jails, but very few long-term prisons. People either suffer some sort of corporal punishment, up to execution. Others are forced into servitude to pay off their sentence. Many crimes are punishable by fines or taking away property.
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?
This varies widely depending on where you are. In most areas there is little difference between a noble and a successful business person. Because of the current timeframe, some noble families are trying to hold onto their old titles, but some are fading into irrelevance while others gain more power with new noble families coming to the fore. In part I did that so that PCs have more options. They can be of noble birth, but the family is broke. They could be trying to regain the glory lost, or establish themselves as leaders.