An eight-year-old made the decision
Twelve publishers turned the first book down. The thirteenth, Bloomsbury, only looked at it because the chairman gave the opening chapter to his eight-year-old daughter Alice to read over lunch. She came back and asked for the rest, then kept asking for months. That is, fairly literally, why the book exists.
They printed 500 copies, roughly 300 of which went straight to libraries, and paid an advance small enough that the author was advised to keep her day job. A surviving first edition from that run now sells at auction for tens of thousands of pounds — and most of the 500 were read to pieces by children in the 1990s, so very few survive in any condition.
The initials were a marketing decision
The publisher's view was that boys, the audience they most wanted, might not pick up an adventure story written by a woman. So the cover went out under initials rather than a first name. The author had no middle name to supply a second initial, and borrowed her grandmother Kathleen's.
It is a small thing that tells you a lot about publishing in 1997, and it was not unusual — the Brontë sisters had done a version of the same thing 150 years earlier under male pen names. Within three years the strategy was irrelevant, since the name on the cover had become one of the best known in the world.
The rules are what make the magic work
Fantasy is easy to write badly because anything can happen, and a story where anything can happen has no tension. The Potter books avoid that by being unusually strict. A WAND chooses the wizard and works poorly for anyone else. A SPELL has to be pronounced correctly. A PATRONUS requires a truly happy memory, so a frightened person cannot produce one.
The same discipline runs through the world's machinery: a HORCRUX costs you part of yourself, a POTION takes a month to brew, gold cannot simply be conjured, and nothing brings back the dead. Characters lose because a rule holds, not because the author needed them to. The limits are the plot. Once you know a MUGGLE cannot see HOGWARTS and a BROOMSTICK will not outfly a phoenix, the danger in a scene is real, because the reader knows what is and is not allowed.
Everything in the grid
The people first: HERMIONE, the WEASLEY family, HAGRID with his questionable pets, DUMBLEDORE, SNAPE, the house-elf DOBBY, and VOLDEMORT, whose name most characters will not say. Then the places and institutions — GRYFFINDOR and SLYTHERIN, the AZKABAN prison, the GOBLET of Fire.
And the everyday texture that made the world feel lived-in: QUIDDITCH with its faintly absurd scoring, homework, exams, a WIZARD bank run by goblins, and the fact that the school's chief problem in any given year is usually administrative. The books work partly because the magic is treated as a subject you can be bad at — there are essays about it, a syllabus, and a teacher who clearly does not want to be there.