I re-read the first three novels last week, and have started the fourth. Observations:
* It is a fast and easy read, it feels like I'm zipping through in no time.
* It feels like we barely see Quirrell, especially as a teacher. But then, we see very little of any classes in the first book.
* Arthur Weasley seems feckless as a person and silly in his ignorant Muggle-enthusiasm but he's pretty competent as a wizard, making a flying TARDIS car, and casually repairing Harry's glasses.
* Rowling's naming wordplay is still great. Yes, it's 'childish' compared to Tolkien or Hodgell, but it works, and there's so much of it.
** "Quirinus is also used as an epithet for the Roman god Janus"
* I remember thinking Harry/Ginny came out of the blue when I first read the 7th book, but Ginny's crush on Harry is pretty starkly obvious from book 2 on. Harry's interest, I dunno. I still favor Harry/Luna
* People often say the Wizarding World is stagnant but there are a lot of counterexamples. One early one is broomsticks, which get better and better over the course of the first few books. Possibly too much so, for something they've been using for centuries. Also, the werewolf suppression potion Snape make for Lupin was a recent discovery, postdating their time at Hogwarts.
* What's with Crookshanks the intelligent cat?
* Owls have some convenient deep magic to be able to find people otherwise in hiding but not be abused to reveal their location or deliver a letter bomb. Well, maybe you could do the latter. [Edit: reading the wikia, there is in fact magic you can use so owls don't find you.]
* First book is 350 pages, 4th is 750. :O
And finally... so, more obsessed minds than mine have grappled with the Wizarding World demographics, but some things leapt out at me. It's very explicit that there are 5 boys in Harry's Gryffindor year, and 20 broomsticks in a two-House flying class. Assuming uniformity, this points to 40 students a year, 280 for the whole school. Given the number of teachers we see, and that one teacher will teach a subject for all seven years, this fits.
Assuming an average lifespan of 150, that'd be 40*150 = 6000 wizards in Britain. Maybe up to 12,000 if you assume severely damaged demographis due to Voldemort.
IMO this fits too. 6000 is a large town or small city by medieval standards, certainly capable of supporting a fair number of businesses, especially given that wizards are quasi-post-scarcity in mundane ways. They don't have an actual city, but with Floo and Apparate teleportation they can basically be a distributed city. The economy is Vague but being based largely on doing magical services for each other fits. Having a top-heavy government for the population kind of fits; you've got a heavily armed population with a lot of free time and a lot of secrecy, leading to high regulation and high "keep them busy". No idea how that's paid for, though.
Some oddities though. 200 people show up on Slytherin's side in a Quidditch match, but maybe they were from outside the school.
If most people don't break their wands much, Ollivander's main business would be supplying 40 students a year. But he's best in the world, maybe he gets a lot of international business. And if 1% of British wizards break or lose their wand a year, that'd be another 50+ wands a year.
6000 wizards in Britain implies 600,000 in the whole world. 100,000 showing up for the World Cup would be 1/6 of the population! But again, middle-class population with teleportation.
JKR apparently has opined 1000 students at Hogwarts and 3000 wizards in Britain, which is an insane population distribution. 1/3 of the population would be in the 11-18 age range. That's not a high life expectancy. Having gone to a 900 student college, I'll say Hogwarts does not feel like that to me.
I'm assuming a modern age pyramid, stable population, tapering off in the mid-100s. One fan disagrees
, noting that wizards died of Dragonpox, and suggesting that while wizards have great healing magic when it comes to injuries, they may be subject to diseases, magical or even mundane (how many are vaccinated?) As well as a rather higher death rate from violence, what with being a population of gunslingers, Voldemort being the most feared Dark Lord of the century... implying more Dark Lords. (We *know* of Grindelwald.)
* How do they get food and raw materials? You'd think they magic it up, but the last book sai they can't live on conjured food, IIRC. Though magic could steal food or tranform biomass, I'd warrant.
* Do pure-bloods like the Malfoys and Weasleys, or Dumbledore, even exist as far as British bureaucracy is concerned? Harry and Hermione should, 'just' living a rich secret life, but what about others? Do their houses exist on Muggle records of title, or are they all mentally invisible to Muggles? We're told Hogsmeade is the only all-wizard community, implying everywhere else wizards have Muggle neighbors, but their deep ignorance of Muggle life belies that. The Malfoys have a manor, but the Blacks had a "don't notice me" house right in London.
** Likely this simply doesn't stand up well to critical thought.