One thing is definitely the worldbuilding; Rowling didn't really even try to do unique worldbuilding beyond wizards (and even there she used a lot of existing tropes), but co-opted pretty much all existing fairy-tales into the universe, with a small twist explained by Statute of Secrecy and misconceptions by muggles. It immediately created an almost impossibly huge universe based on all previous fantasy, and that allowed imagination to really go wild.

It's almost ridiculously easy to imagine that Narnia actually exists inside Harry Potter universe, and that the wardrobe was just some weird experiment by some eccentric wizard, and the books written by C.S. Lewis are based on a distorted memory remaining after botched obliviation...

 

There are many elements intrinsic to the books which made the HP series sucessful:

- Very accessible writing.

- Page-turning plotting.

- Simple but distinctive characters.

- Good humour.

- The sheer charm of the world, which makes you want to live there. Ultimately, Harry Potter is one of the few fantastical universes where you would actually want to go - most of them are far too grim to risk it.

- Magic which more represented how the typical person thinks about magic (in a fairy tale manner) rather than what the traditional sci-fi/fantasy crowd were used to (i.e. a magic system which can be off-putting to casual readers).

However, while these all contributed to the book' success, none of them are sufficient to explain just how amazingly successful the books were. That is in fact a result of the context the books found themselves in, which cannot be replicated. These include:

- Bringing the "boarding school" genre to a new generation who (apart from the small number of enthusiastic readers) were not familiar with stories like Malory Towers and St Clare's.

- Bringing fantasy to a large number of readers who had previously ignored it. In many ways, Harry Potter was the start of the movement of nerdiness into the mainstream, which Marvel and Game of Thrones have continued.

- Bringing urban fantasy to a large number of readers who had previously not encountered it. Prior to Harry Potter, most fantasy was of the high fantasy type, set in another world. Urban fantasy of course existed, but it wasn't a significant part of the market. The juxtaposition of magical and Muggle was novel to a lot of people and added to the "you want to be there" aspect. It was so easy to imagine yourself in the world.

- Excellent marketing which turned the books from a success to a phenomenon.

- The "Facebook effect": once enough people had read the books, other people ended up reading them just to fit in.

- The way the release of the books was spaced out, resulting in anticipation building between books and allowing people to grow up with the characters.

 

I would say the timing of it was also pretty much perfect. The start of the internet and its global cultural impact but before it consumed all other forms of media. The number of American adults who haven't read a single book the last year has gone up by half since the last HP book came out.

 

Saw a really disturbing Jimmy Fallon segment. Think they were in california.

Person went around asking people to name a book. Any book. Most couldn't. One person replied,

"I haven't read a book since high school.'
It was said with pride.

Quite disturbing.

 

To me, it was the simplicity of the series.

Most of the magical world mirrored the muggle one that all readers knew. There very few truly magical concepts. The ministry was the government, brooms were like bikes, Quidditch was like football, etc. It's why the world building worked so well; rather than building a world from scratch Rowling merely added on to the existing one.

Plus, most other fantasy stories tended to introduce more esoteric concepts like fae/fairies, necromancers, special types of magic, etc. These concepts are usually a bit hard to keep track for younger readers. The closest Rowling got were the Hallows and Horcruxes.

Let's also not forget how relateable it is. Sure, it's set in a boarding school but it mimics even public schools a fair bit. Most kids always have that one teacher that everyone hates (Snape), they rarely ever see their headteacher (Dumbledore), they usually have two close friends (But usually a wider circle of less close friends).

 

Was gonna answer then read what Taure wrote and don't really have anything left to say.

What makes it compelling from a fanfiction angle is that the world building is delightfully flawed. Rich enough to build off of, but with plenty of holes to fill in yourself. If Rowling had gone the route of as Taure says, a magical system, or tried to develop an answer to everything beyond, 'it's magic, bitches', I think the fandom would be much impoverished.

As is, the books gave us a glimpse of a deeper world, but canon is seen entirely through the eyes of a not-entirely-trustworthy narrator with limited interests in some areas and exaggerated ones elsewhere. It's a perfect construction site.

 

On a related note:

 

To me, it's all about Hogwarts. She took the classical four elements/four humours idea and put it in a school where you get to learn magic. That's truly a brilliant idea, and its why people still argue about which houses their friends, celebrities, characters from other forms of media, etc. would be a sorted into.

Just the idea of going to a special school to learn to use cool powers is one of the reasons the X-Men is still such a huge comic series-- especially if you remember how that particular trope saved the movies with First Class after how terribly received Last Stand & the Wolverine origins movie were.

But the 4 houses is the simple thing that puts Harry Potter over the top in my opinion.