-- I've always liked the fantasy/magical reality genre (provided it's well-written).
-- JKR is a fantastic writer. Her words are well-chosen, and she rarely wastes a word. She draws on older forms of literature -- the English boarding house novel and the fantasy/magical reality tale -- in a way that demonstrates her complete familiarity with what has gone before, but that uses existing elements with utter originality to create a compelling story of good vs. evil.
-- JKR has an almost Dickensian gift for naming characters. For example, Draco (Latin for "dragon," which in medieval understanding is considered a big snake, perfect for Slytherin) Malfoy (Old French for "bad faith"). Or Argus Filch, the caretaker who keeps an eye on everything at Hogwarts and who is named for the 100-eyed giant of Greek mythology. Remus Lupin's name should have given away his secret early on -- I could have kicked myself for not catching it.
-- The book is full of characters that one cares about and that are fun to hate.
-- They are smart books, with many levels. As some have already noted, JKR deals with some serious issues without preaching.
-- If you know some Latin (or a few other languages), it's fun to decipher the meanings of various spells and incantations. (If you don't know Latin or these other languages -- she even uses Aramaic -- check out the Lexicon at
http://www.the-leaky-cauldron.org/.) If you know medieval literature, you can pick out all sorts of symbolic elements.
-- How can one not like a book with
Ernie Bott's Every Flavor Beans?
-- They are
great fun to read out loud, which my 5-year-old demands on a regular basis. (He is convinced that when he turns 11, he'll get a letter telling him he has been admitted to Hogwarts, despite being Muggle-born. He already says he wants to be Harry Potter for Halloween -- I'm trying to convince him to be a little unusual and be Harry wearing Quidditch robes instead of school robes.)