AbdulAlhazred
Legend
Honestly, JRRT's world building never impressed me in the slightest. Between the Declaration of the Doom of Mandos (which comes at almost the same time that the Sun and Moon first appear and thus the start of real timekeeping in the First Age) when the Noldor returned to ME and the Fall of Sauron at the end of the Third Age is something on the order of SIX THOUSAND YEARS, and yet only 3 human nations exist amongst the Dunedain in all this time, and they can trace their individual ancestries back all the way to men like Hurin and Turin (or at least their generation). Vast periods of time fill just the Third Age, 3000 years, in which basically NOTHING happens, society is in complete stasis. There's NOTHING realistic about the history of ME, nothing at all!That's more a statement that ME's consistency is not complete, and that was in line with what I understand of Tolkien's focus. He says that he was concerned to ensure geographic, chronological and linguistic consistency. He did not always succeed, but he was hugely helped in those goals through his prior world-building. His books would not be the achievement they are, were it not for his world-building.
It has no realistic (or really any) economy, very little society (most areas are simply lawless wilds which seem to remain so throughout the entire period), a completely static technology, etc.
In fact it is the very AVOIDANCE of all of these things which gives ME its mythic abstract character. It is NOT really a living breathing world, its a sort of diorama. Its a bit like a train set, the trains go round and round, but nothing else ever changes. The Shire is the exception, and it is no coincidence that all the really humanized characters and details of everyday life are pretty much drawn from that one location. The Hobbit and LotR (some parts at least) are VERY different from the rest of Tolkien's mythic work, and required their own little reservation to inhabit.
This captures quite well why GNS comes under criticism for turning a blind eye to some kinds of roleplaying. I would call stories set in the Wild Card setting "shared fiction" and yet many details of that setting arrive to authors as part of the world backstory. The point being, participants can share creation of fiction in a setting that was created by a subset, or none (!) of those participants. Say we choose to play in an authentic 5th century Roman setting? None of us create that setting, but our fiction is shared. We might add some fantasy to it in specific ways. Indeed, to the extent that concepts pre-exist or arise in the minds of some and not all participants, the fiction is never shared in the sense you want.
Possibly that is because the sense you want is very purist, and it for me isn't fully admitting what is going on. If my character nominates a fictional manufacturer of her fictional grav bike, then other players should accept my fiction. They shouldn't say - no, that manufacturer doesn't exist so your character cannot be sitting on her grav bike, that said manufacturer putatively crafted. Thus, they accept something that they had no agency over. This is a constant. The only question is the scale and siting of who is doing what.
If someone comes to my group with Barker's EPT and we feel excited about that world, we can still weave our own tales into it, with full agency over our part, our fictions, without needing to have agency over the details the world-build provides. Or in our shared world, I might be obsessed with the cartography, while Alice is obsessed with the politics: a shared world-build is still a world-build.
And if we come down to - is the question really about whether a world-build can be shared? Then sure, of course it can. But not everyone wants to do it, and even when they do they frequently have different parts they are interested in. The value of the world-build remains the same. For shared fiction to work, each player must accept the fictional contributions of other players, and all must accept historical contributions they have chosen to rely on, and if that is more on one player, a DM, and less on others, the P/Cs, that is fine. It cannot be tarred "classic" DnD and mildly denigrated as "puzzle-solving"! It can and should be narratively rich.
All of this is great, but what is the common reality?