I find Rowling's language completely appropriate to the style and tone of the book. Do I find Fowles a better writer, though? Yes, because he engages me more. When I read Fowles' work, I suspend my disbelief emphatically, and embrace his characters and style as fictional embodiments of myself and my own world; I see many traits in Nicholas Urfe which I find in myself. When I read Harry Potter (which, ironically, was for school, about four years ago), I also suspended my disbelief. Which is, I think, good enough; especially for a fantasy novel.

Rowling is a far better writer than Puzo, I think.

To be fair, though, my thoughts on Rowling's language stems from my (vague) memory of it; if I revisited it today, I may well find it atrocious.

I know many girls who cried at the newest Harry Potter novel. If that doesn't merit praise for its use of language, I don't know what does. You need some kind of linguistic skill to evoke that kind of response in your readers when you're wanting to.