I am absolutely addicted to both His Dark Materials (I have never spent so much time alternately crying, laughing and screaming in rage at the author of one book) and the Harry Potter series, and I can understand why both of them are popular--and why both appeal in different ways.
Let's face it--HDM is an incredibly richly conceived, deep philosophical work (based on Milton, for gods' sake!), while HP is an entertaining and compelling read about a group of characters that many of us can identify with.
It must be faced that, despite all the great literature out there, a lot of people don't read to illumine their intellects or challenge themselves--they read to lose themselves in a different world. And there's absolutely nothing wrong with that! That's no moral judgment on anyone; it's just a fact. I get really frustrated when my "intellectual" acquaintances turn their noses up at my favorite authors (Rowling/Pratchett for example) because they are more popular than Pullman or
. Just because Terry Pratchett's Discworld novels aren't going to be nominated for a Pulitzer any day soon doesn't mean they don't make me laugh and cry and question and think deeply (he actually faces a lot of serious issues in his own deliciously sarcastic way). He doesn't write classic literature, but he's incredibly clever in getting his message across. Joanne Rowling probably won't win a Pulitzer either, but she is very skilled at writing such fun, likeable (and hateable ;)) characters that half the civilized world seems to have become addicted to them. There's something to be said for that.
P.S. This post isn't meant to disagree with or challenge anyone else on this thread--I think everyone here has expressed themselves quite well, and I know no one has really attacked J.K.R. or anyone else. This is just one of my pet subjects. Snobbery is snobbery, and I'm as guilty of it as anyone else, but just because great literature exists doesn't mean that works of "lesser intellectual merit" shouldn't. Of course, truly bad writing also exists (and I'm the first to make fun of it *grin*), but I generally don't rate truly bad writing by comparison with anything except other truly bad writing. :)