I haven't read the books but I probably will have to read them at some point. I have sorta seen a couple of the movies but they didn't make much of an impression on me. What I was wondering is why does everybody think the series is so exceptionally popular?
I gathered by comments that the books contain neither exceptional writing, nor seem particularly original in characterization, dialogue or plot and yet the books and movies have had amazing success, and the creative outpouring and perversion of the fanfiction is astonishing.
-Very easy to read and understand - written simply, not quite condescendingly.
-Easily-identifiable stereotypes - there's no danger of deep, dull characterization when you know well each role each character plays and will continue to play, excepting the occasional SHOCKING TWIST.
-"Fun" fantasy with high magic - you have examples of magic that resemble technology we don't have (the trading cards with wizards that move and do things like they're tiny videos), examples of magic that have special altered technology that would just be super-cool (flying car, flying brooms), and magic that's just magic (pretty sparkles).
-Strong enough concepts of "good" and "evil" that many people can project much of their own personal philosophies onto the books. How many "it's okay to read HP, Christians!" books are there? (probably more than the "HP = devil" books)
-Male lead, so that everyone can identify with him.
-Lead who had a crappy life, but then become special and famous because of circumstances of his birth he had no clue about - maybe you could get lucky and end up whisked away to a magical land because you're secretly Chosen!
Of the kidlet series I mentioned, several featured ambiguous morality - there are still concepts of good and evil, but many of the villains or less-likeable characters are more fleshed out. Most had female leads, which means that boys are going to be much more relucant to read them. Most preferred archetypes to stereotypes, and when either character-type was used they tended to be fleshed out. One required knowledge of other fantasy works. One has fairly stylized writing, which, though not hard to read, may be offputting or confusing to some younger readers (they might actually miss the point of the character's pecular narration).
I haven't the slightest clue why the HP books are so popular with adults.