My friends who are librarians have commented that she seems to have made reading much more popular with kids. That's worth something in itself, whether you like her books or not. (I like her books pretty well, though only Prisoner of Azkaban ranks in my "favorites" list.)
Also, at least one other author who publishes in the "young adult" market has been grateful for the ability to write and sell longer books for kids, now that Rowling has proven to publishers that kids will buy and read longer books. So that's a plus, too.
As far as originality and general writing quality goes, she had some well-conceived characters, and more backplot than a lot of authors seem to bother with. She wasn't always so great with logical consistency or (IMHO) pacing. But I did like how the portrayal of the adult characters and the world in general changed as the main character (and presumably the readers) grew older and matured over the course of the series.