I am a HUGE Harry Potter fan! I only started reading them about 2 years ago while working at Borders and wondering why so many people were buying these kids books. Needless to say I was hooked. What I like about them are the writing quality as well as character development. The description of the places and what is going on makes me feel like they really do exsist - but that's a good thing for any fantasy literature, right?
I feel like I'm a part of the story and the character development really allows you to get to know the characters.
Those are a few of the reasons why I like the books so much.
Edited to add: Ditto to what Lady Pi Phi said
Sarah