I think some of the Harry Potter following is faddishness--it is just barely good enough that most adults can read it and think 'that was okay' by itself. A lot of these same adults have insulated themselves from wonder in many respects, however, and particularly fantastic wonder, so it comes across to them as something new, whereas we fantasy and scifi nerds are like 'eh'.

The reason I say faddishness is because children's librarians (like my mother) have had problems trying to get kids to cross over to other totally awesome children's fantasy that's out there, and there's really a lot of it. You don't have to work /quite/ as hard at world-building when dealing with young audiences.