I think part of it (other than fabulous world-building, numbers aside), is that it hit a chord. Something about Harry Potter and Hogwarts gave us that "whisked away to fantasy land" feeling that we hadn't really gotten since . . . . Narnia or Wonderland. Or Tolkein, but he's more for adults or at least teenagers.
The circumstances around the book also had an impact. The idea of this single mother writing this wonderful book on napkins because she couldn't afford a computer and all that "little guy making it big" thing that we love. So people heard that, heard the book was good, and decided to give it a shot. And here we are.