Her prose/ style is considered to be very restricted. To an extent, what counts as good prose is subjective, although a consensus can usually be reached. Her writing suffers from overuse of adverbs (""never met an adverb she didn't like"). Her vocabulary is thought of as limited. Her reputation is that of impressive story-teller but inadequate writer. Sort of like an average writer who had stumbled upon an amazing idea.
As for world building itself, it seems interesting on surface but if you dig deeper, some of the things don't add up.
"For example, why would the magical world hide from muggles and how did the wizards manage to convince all other magical creatures to do that? How did every magical culture come up with idea at once? Surely there are some cultures where magic wasn't hidden. How would they even begin to do that? "
The whole muggle-wizard interaction aspect of the story was very poorly done and this is one example.
Below is the article from Harold Bloom on this subject. He is the foremost American literary critic. His views quite resonate with bibliophiles/ scholars/ professors.
http://archive.boston.com/news/glob...les/2003/09/24/dumbing_down_american_readers/