Don't get me wrong
I think it was Apollo Creed who said of Rocky "he fights great but I'm a great fighter."
Don't get me wrong, as I said, I think she has a very vivid imagination and her books are a lot if fun to read. But I would not consider her to be a great writer in the class of Tolkein, Lewis Carrol, Dahl, or others whose works appeal to younger readers. If you read Tolkein, he has an absolute mastery over his subject matter and is technically superior to Rowling.
I'm not trying to over-analyze this, because she is fun to read. It's also terrific she can get people to read more. I just don't think she is in the class of the greats such as Faulkner, Hemingway, Fitzgerald, Melville, Dickens, etc. I simply find her prose, not her story telling, to be average.
I just kind of get the feeling that she's like a Milky Way bar -- great while you're consuming it, but a short time later you're hungry again. Tolkein is like eating a whole meal. To paraphrase Charles Barkley: I might be wrong, but I doubt it.