The main appeal in Harry Potter seems to be that "It gets kids to read!"
Since I couldn't care less about kids, well... *grins* HP isn't that awful, though. It's light and fun, and... sellable, I suppose. Has all the elements people want to see. Y'know, Marion Zimmer Bradley wrote an article about publishers. She said that they don't pick manuscripts that are well-written; they just pick ones that sell.