The difference between Harry Potter and James Bond is that every one of us has seen a James Bond movie that just plain sucked, and we said to ourselves "god damn Bond sucks now. Let's not go see another Bond movie for like 10 years." "The Living Daylights," anyone?
The Harry Potter movies have been remarkably good, and I think it's because they're humble. They don't really think they're making classics. They're not trying to define a generation. They don't pretend they're making art films. They're just kiddie stories meant to entertain. They set their goals a little lower and they end up succeeding quite nicely every time.