In an RPG sourcebook, the information given is generally about System or Setting.
System is the hard rules about how to resolve questions in the game; what dice we roll to determine success or failure.
Setting is the information about what the game world, and adventures in it are supposed to be like.
Theoretically, we should be able to separate the System from the Setting. We should be able to take the system rules and apply them to a variety of settings.
What I find with D&D however, is that the System-as-provided dictates huge amounts of the Setting. They are practically intertwined. It imposes huge assumptions about the Game World and what goes on it.
As soon as players plug into the Race/Class matrix to build a character, we have Assumed that all these various types of people exist and have some function in the world. The DM then gets to arrange this motley crew into some sort of "party." The simple Adventuring Party is a D&D trope unto itself.
Then there is the Magic as listed in the book. As soon as we have a list of Spells, and magic-users calculate what to burn their spell slots on, we have a world where magic isn’t really all that Magical, but more like ammunition to be managed.
How about that spell that returns people from the dead? That makes for a world which is fundamentally different from any human reality.
How about other spells which summon energies and entities from other Planes of Existence? D&D simply assumes a Cosmological Circus. I also find that under these circumstances, playing a magic-user requires a significant dollop of genre-savviness in order to feel effective.
And if a Gnome is a 3-foot-tall magical person as described in the PHB, then what do we call the creatures that are 3-inches-high with red conical hats and keep mice as pets?
Then you have the Levelling System. It is nice for a character to grow and gain power. D&D has a conceit of character level, and facing PCs against roughly equivalent enemies. This creates the assumption that PCs will begin their careers by slugging rats, then eventually become powerful enough to tell God how they really feel. This creates a serous expectation of scope for Players and DMs.
At the end of the day, we get a Bizarro World where, perversely, a Dungeon is a place people Want to go.
All this about the Assumed Setting of D&D is sticking in my craw lately. Partially because I get to teach it to new players, and I find it is a whole damned thing. And partially because I have game ideas which are decidedly Fantasy, but would not fit into D&D as D&D.
D&D could be a game about Anything. The source books have everything from dinosaurs to Hellenic deities to mind-devouring space aliens.
However, the tendency in play has been to make it a game about Everything. The Forgotten Realms setting (which includes locations like Baldur’s Gate) is probably the definitive, official D&D setting. It is definitive because it makes room for All the Usual Stuff, and whatever else has been published in official books.
(Parenthetical statement about content bloat, Hasbro, cranking out books and crap, commercialism)
It’s a great body of work and imagination. But all the elves and warlocks and displacer-beasts tend to choke out the notions and visions (delicate and fragile ones) which will be unique to each DM and group.
If a DM wanted to cut their game loose from any of these assumptions, they would have to brutally pare out huge sections of the rules; The D&D parts that Players probably expect to be in a game of D&D because that’s what D&D is like. That's what I'm toying with.
I am thinking about trying other game systems, or taking a knife to the Rules of D&D. But that’s a separate post. I’d like to maybe run Mage: The Ascension, just to show my players that D&D isn’t the only option for tapping that TTRPG magic.
For now, all of this is just to explain to my players that D&D is its own beast. Much of the experience is guided by its Assumed setting, and maybe not so much by our own imaginations and fancies. (Especially in these current adventures which are specifically meant to teach the Assumed setting) Please take it with a grain of salt.