Her prose is suitable for both children and adults so it's not flowery, but not boring either, I always find myself turning the next page at the of the previous one. It's a genre-definer, particularly urban fantasy because it mixes fantasy with realism, even though the magic isn't realistic, the world is; it's wizards that live in a world like ours, so it's more relatable than, say, Game of Thrones. Harry is also a very troubled kid, and the more he is, the more relatable he is, J.K. does a great job making us root for him, you just can't help it after what he's been through and what he's after.

Overall, unique and realistic characters, deep themes, distributing many feelings throughout the series from sorrow to happiness, funny to despair without forcing it, good handling of the "chosen one" trope, good fantasy setting, interesting and relatable main character (this is very important because the entire series is through Harry's point of view).

To sum up, Harry Potter does almost everything right, is incredibly relatable, and it caters to a huge audience, and is unique, a genre-definer like Lord of the Rings, that's why it's so successful.