I could open any book to a random page and start reading, and I'll bet I'd have a story idea within three pages. I might describe it as "here's something else JKR screwed up," but it would be a story idea!

Ha! Also: same. I started the series when just the first three books were out, and I picked them up entirely because they seemed to have fannish potential. The online fandom was still small enough to qualify for rare fandom/ship mailing lists, and the appeal for me was a charming, inventive world aimed at kids but with a large number of interesting older characters and lots of kid-lit gaps to fill in as a fanfic writer. I was disappointed by many aspects of the later books from a storytelling perspective, but one of the things that jostled me out of the fandom a little as it became such a huge phenomenon was the contingent who felt that HP fandom should only be for people who thought the books were some of the best they'd ever read. As you say, the characters went in some directions I don't think were well thought out or earned, but here I am still thinking about them and wanting to play with them all these years later.