I've been reading and writing in many fandoms over 22 years. I've been reading Harry Potter only since 2000, when I got bored in New Zealand and finally broke down and bought the first two at a bookstore there. It has sparked an interest for me in traditional myth structure and hero characters.
Harry Potter has held my interest (obsession?) because there are so many characters that can be developed and taken in whatever direction I feel a particular story needs them to go. Most fandoms have a lot less freedom to develop alternative threads and many fewer characters.