In other words, what's the meat and potatoes of your story? What's the subject matter? What's the substance readers will sink their teeth into? What's the world you're building and why?
In Pullman's The Golden Compass, there are multiple worlds.
And I have a gut-feeling that, in the initial world of one Lyra Belacqua, Pullman did not end up with polar bears and 'gyptian' people and daemons and dark matter and 'Dust' and Oxford in a parallel universe and Muscovy and Arctic traverses and auroras and zeppelins and witches and a Texan in an air balloon and the Magisterium and other subject matter altogether in a rehashing of Milton's Paradise Lost (remember to inhale) by mere coincidence.
No, Pullman must have either began with a laundry list of elements, or (probably more likely the case) began with one mesmerizing element, say Oxford in a parallel universe, and then grew his world from there. Gobbing on pieces perhaps willy-nilly until he had an unformed mass of clay to work with, which then needed scraping, adding, removing, and finally firing to reveal its veneer and firmness. A world combined from many disparate elements yet appearing as one congealed whole, such that it's impossible to detect the hairline fissures between pieces.
Pullman's world is doubtless a construction which serves a purpose. That's his authorial intent. Author's made-up worlds, like their characters, serve the stories they want to tell.
The art of world-building, for especially fantasy and sci-fi authors, is a skill in and of itself. Your world has to achieve a lot of things: authenticity, a sense of magic or chance or opportunity, a setting for characters, an interaction, a believability.
The trick may be to have many varieties of potatoes and many varieties of meat; too pack on the elements. But not too many. But then again when is enough enough? When do you add more elements to fill-in this plot hole or that backstory? When do you take away elements that are just overkill? These questions can be answered with practice, with reading what other authors have done, and with feedback from readers.
But, in truth, there may be no 'learning' what that magical spice is which adds just the final flourish to a world to make it perfect. That nameless something which separates great fictional worlds from okayish ones. That answers a profound yes to: Does your reader want to inhabit the world you've created?
Think about how many readers of Harry Potter, to this day, even as adults, still want to escape down Diagon Alley, or through the pillar in King's Crossing, to Hogwarts itself? J. R. R. Tolkien spent his whole life building the world of The Lord of the Rings. How long have you spent, budding author, building yours? How much do you want to escape to that place yourself?
That said, Pullman's writing style comes across as more than a bit fancy-pants with semicolons galore; otherwise, good read. Good world.
Look for my next post on The Subtle Knife!