When I teach creative writing modules, I use HP as what not to do.
Clunky dialogue
Plot holes as big as the one in the ozone layer (none of the kids from wizarding families knowing that Tom Riddle was Voldemort's real name? That would be like kids in "our world" not knowing that Mr Claus's name was Santa.
Her sentence structure is predictably wooden Every noun has its adjective, every verb its adverb. And they are invariably the same ones throughout the saga. Now, I know that was how it was done in "rosy-fingered dawn" days, but I'm sure JK has a wider vocabulary than "oh, they're running! Must add a "breathlessly" there!"
Etc etc.
Name repetition instead of pronoun use.
The list of stylistic clunkery is endless. I don't blame JK, but she needed a better editor and she needed one to actually pull her up and tell her that occasionally, less is more . The first three books are fairly well-written for the age group they were aimed at, but once she started the two-year churning out of doorstopper volumes, her writing went haywire.
Don't get me wrong, there are wonderful bits, fabulous. But she needed to plan better, (in order to avoid the millions of questions she had to answer to explain things that don't make sense, we've all seen and read the "ah, but she wanted it to look like that really!" stuff) and she needed more time.
Her name choices are inspired however!