There are two issues that came up in the course of that thread, which weren't necessarily *directly* relevant once they started growing.
--Population of the wizarding world.
--How the wizarding society functions (or doesn't); a subsection of this could be discussion of the shortcomings of the Hogwarts educational system as seen in the books.
I'll start off with a quotation of a post that I wrote in that thread regarding the numbers of the student body at Hogwarts, which derived from an estimate up-thread that the wizarding population was likely ~20K to 30K people in the UK:
But the population issue does raise the question of how many magical kids *are* going to Hogwarts. If you've got ~20-30K wizards (full-blood, mixed, and one-offs like Hermione) in the UK, most of which are married or related to each other, and say... a generous one-quarter of that is kids... that's a *lot* more than we ever really hear about being at Hogwarts.
Let's see... a quarter of 20K is 5K... quarter of 30K is 7.5, so between 5,000-7,500 children. Hogwarts doesn't start till kids are ten, so let's be generous and halve that... call it ~3,000 children. Four Houses at Hogwarts, and each one is probably roughly equal in number (nobody ever says there are more Hufflepuffs than Gryffindors, for example), so call it 750 kids per house. Divide that by seven years, 100 (and change) kids per year per house, so that's ~400 new kids each year (and of course ~400, minus wash-outs, leaving at the end of their 7th year).
Again, that would be ~3,000 students at Hogwarts each year in total. Even knocking it down a bit to allow for a low birth-rate, it would still be between a thousand to two thousand. Rowling said a while ago that it's ~1 thousand, but neither the movies nor the books support that. One thousand would be plausible for what appears to be the wizarding world's equivalent of a large English private school like Rugby or Eton. Three thousand is stretching it rather farther than that.
http://www.beyondhogwarts.com/harry-pot ... warts.html
There are only ~20 teachers (minus the rotating cast of DATDA teachers and occasional guest-stars like Firenze) named, including the headmaster. If you don't count Filch and Hagrid, that's a couple less. Unless they've got time-travel shenanigans going on (not impossible) or there's a vast cast of unnamed teachers (also possible, but the series never says anything of the kind), that's simply not enough unless the students only visit each teacher for one class a week. Of course they're not covering more than a few subjects per year, but some subjects you repeat every year (DATDA, anybody?).
The books never really say that there are so many students at Hogwarts; certainly there aren't that many named students (with the caveat that odds are good that Harry wouldn't have known many kids outside his social circle of Gryffindor/Dumbledore's Army students and of course the books are largely from his viewpoint). There are a number of common-room meetings, and unless there's some serious spatial mindfuckery going, I don't see ~750 kids fitting into the Gryffindor common room for McGonnall to give them a lecture on how they're not supposed to go out at night or whatever. Maybe if she's speaking to each year in turn, but nonetheless.
If Rowling had ever said something like "well Hogwarts is just the best wizarding school in the UK, there are other schools that aren't as good but they're cheaper so more kids go there" that would have made all that much easier, but of course she didn't think it that far through. One explanation that does occur to me is that you have to qualify at a certain level of magical ability to enter Hogwarts, but the existence of Squibs who attended for some time before washing out disqualifies that (unless they were charity cases). If it's a financial thing-- unlikely given the Weasley family are dirt-poor but they manage to send every child to Hogwarts-- that would be another possible explanation. Last idea that occurs to me is that homeschooling is really popular among Wizarding families? I got nothing...