I have recently had an alpha reader asking me details about something, and I ended up noting that yes, there’s a whole larger reason and history that would build into why that detail was, “but that’s another story for another time.”
Fortunately, the alpha reader didn’t take this as reproof, but cheerfully noted that while it was a minor bit of scene dressing not relevant to the story at hand, the desire to know all about the history and the greater world was not actually necessary to the story, and just a personal itch they’d like to scratch.
In discussing stories with another writer, she mentioned how my stories were fully built worlds, while another author’s world was “thin.”
I assure you, dear readers, my worlds are not fully built monstrosities with world bibles and deep histories prior to the characters walking on stage. More than once, what seems like thorough world-building has just been me making an in-joke or reference to an old pulp story or a song I like (or sometimes even a meme), and then having to sit there and think “Now that I’ve put that in, it’s funny, but how do I justify this in context of the world? What would have to exist for this to be here, and how would that affect the story?”
More than once an alpha reader… sometimes a beta-reader… G-d forbid it be my Calmer Half as he’s doing the copyedit prior to publication… says “Why?” or “Where?” or “How?” and my response is “Uh…. I don’t know. Let me go make that up.”
Now granted, I do try to make geological, geographical, and ecological sense, much less economic and realpolitik… and of course, stay tactically correct. But that’s just continuity, to make sure the world doesn’t break on some internal contradiction, nor break the reader’s suspension of disbelief.
But I’m affected with that blindness where it makes it really hard to look at my own work from an outsider’s perspective. So I’ll just ask you for examples.
What is a book that feels dense and the world lived in that you enjoyed? Why did it feel that way? How?
What is a book that felt thin, but you enjoyed anyway? Why did it feel thin? How?
Mad Genius Club 
Was doing my usual beta read and copy edit combo with a good bit of back and forth with the author and made the observation that the main character seemed to me to be somewhere in the high function spectrum. His response: “good catch, I based the character on one of my own kids.”
Had to hand it to him for drawing from life to make this character feel real.
A lot of the classic, hard sci fi of people like Azimov and Clark felt thin. Their focus was on the technology and how people reacted to it, not the background of that tech or of the people unless it played a role in the story. At the time, I enjoyed the books (original Foundation, The Gods Themselves, Rendezvous with Rama, A Fall of Moondust), really liked their short stories, but the worlds were thin and very specific to the story. Note: It’s been a decade since I read anything besides short stories from either writer, so I could be misremembering.
I remember them as in-the-moment-invented-world, too. There wasn’t much else out there to compare them to, so I accepted the thin-world/sci-puzzle aspect of the story as normal for the genre and went along with it.
—–
(I’ve mentioned a classic example of thinness elsewhere: “One of my favorite classic science fiction novels is Eric Frank Russell‘s Wasp (1957), where a human in disguise wreaks humorous psychological havoc on an enemy alien world during wartime by traveling through its cities suggesting the presence of terrorist organizations. There’s only one main character, and a great many puppet villains. But that’s alright, because in this entire book, praised for its “gritty realism,” set mostly in urban areas with crowd scenes, shops, and public transport, in a humanoid culture very like our own, there is not one single female character (or reference to one) of any kind, puppet or not. Anywhere. Doing anything.
This is not part of the premise, it just happens. The only time the word “she” appears, it refers to a car. And the hell of it is, you don’t even notice, and I’m not sure Russell did, either. I must have read it half a dozen times as a teenager before it struck me.”)
—–
The “big” fantasies before Lord of the Rings came out were more serious about world-building, but they had other flaws. I vividly remember (8th grade) when Tolkien finally became available in the US how very fascinated I was with the way his world-building wasn’t just deep, but reflected in the linguistics, the changes of words, the distinctions of cultures. Rohan was the example that particularly clicked, since I had some vague prior knowledge of the Anglo-Saxons. LOTR defines spectacular world-building for me.
I never hoped to be Tolkien, and I recognized how SFF as it was flowering couldn’t tolerate world-building on that scale (barring exceptional cases), but I’ve always admired the SFF authors who could most effectively utilize hand-wavium in conjuring up depth of history or breadth of cultures in their worlds.
Then there are the Noddle Incidents. 😆
https://tvtropes.org/pmwiki/pmwiki.php/Main/NoodleIncident
I love Noodle Incidents and I have to remind myself not to put them in my stories because if I start doing so I’ll never stop. Just one is simply not enough and I have no restraint so far as these things go!
All books where the world is in danger in the first chapter. You need a little more time to build. In fact, I would lay down as a rule that you shouldn’t tell me the world is in danger until the middle of the first book, even if it’s the only book. (Some variation if it’s a fat book.)
I reread the Warlock Inspite of Himself a couple years back, and although the setting is comparatively well-thought-out (in the sense of accounting for what it needs to account for), it’s pretty clearly just there to justify the specific set of “science vs magic” hijinks Stasheff wanted to include in his novel. Still pretty good fun. Til We Have Faces is more serious but has a similar vibe of being an erudite man’s thousand-foot-view extrapolation of the world he needs for a particular story he wants to tell.
Perelandra is my favorite piece of C. S. Lewis world-building. Almost a pure travelogue, with a total of three human intelligences and three superhuman ones as characters, but what a fascinating place.
In terms of “completely failed world building,” I’d have to go with Marion Zimmer Bradley’s Atlantis books. The main setting is “The Ancient Land,” and the effort that went into that name is roughly indicative of how much effort went into the world-building as a whole. Characters go to “the Temple” to celebrate “minor festivals.” Despite the fact that the majority of the characters are “Priests of Light,” I have no idea what the Priests of Light believe or what their duties are. The entire thing felt like it was taking place on a minimalist stage set, and you could almost hear the sound of Bradley rummaging through the prop closet whenever the characters needed something she’d failed to provide.
In terms of great world building, Tolkien is the obvious example, but the reason that his world felt dense is because it was dense; he has a thousand years of history sketched out along with the origins of all the major countries and how they related to each other, along with various vignettes about historical characters. A more interesting case to look for might be a world that feels like it’s got a history and a large population of real people without the author putting in so much background effort. What details can you use to create the illusion of a deeper world when it really isn’t there?
I think it was Karen Myers (if not I apologize) who mentioned on another thread that some of her more interesting world-building came from accounting for in-jokes/throw-it-ins. That’s been my experience too.
Stories usually come to me as a broad sketch of character/situation with a vague background attached, and world-building is a mixture of extrapolating from the more obvious stuff I’ve got, and accounting for the less obvious stuff I’ve decided to include on a whim. I invented the indenture chips in Shadow Captain mostly to account for the heroes’ loyalty to an employer who didn’t deserve it, but it turned out to have broader implications in the setting.