I am not a fan of the Harry Potter books. I read the first three, and thought they were fun, but nothing to write home about. On the fourth book, I completely lost interest because the long detailed descriptions of the wizarding world held no interest for me. But all that descriptive writing, which I find so tedious, seems to be what the fans love.
From what I have read of the books, it seems the creative acheivement of J.K. Rowling is not in the crafting of a story, nor in the style of writing, nor in any kind of genuine originality, but in the fact that she created a fantasy world with a vast amount of detail. That's why to one person the books may seem irksomely cliched and "imaginatively derivative," whereas to someone else they are "richly imaginative."
But the way in which J.K. Rowling gave her imagination free reign--although it results in a magical world where anything can happen--is, I think, detrimental to the art of storytelling. Another blog I was reading discusses how an author can fall into the error of letting magic in their stories become poorly thought out, and even inconsistent or illogical. This is increasingly a problem the fewer limitations the author has decided to work with.
The use of magic in a story is much more fun and interesting when it has clear limitations. We see it in all the classic fairytales--the hero/heroine is given three magic objects, each of which can do one thing only; the wicked fairy places an enchantment, with a very specific result, and a very specific antidote (she will sleep for one thousand years ... it can only be broken if a prince breaks through and kisses her). These stories would not work if magic could be used for anything, by anyone, at any time.
This is something I've struggled with in writing my own young adult fantasy stories. It's hard to make the use of magic both fun, and creative, as well as limited and specific, so it doesn't get out of hand. So in that sense, I suppose I have a certain degree of respect for J.K. Rowling, since she did write books where the use of magic "works" for most people--though not for me, I'm afraid.