One aspect of the book, Blue Moon by Alyson Nöel, is the genre. I've always loved Paranormal/Supernatural Romances. This book is more of a Paranormal Romance though.
Another aspect of the book I like is the originality of it. I have never heard of a book series about a girl who is a psychic turned immortal. Originality is a good feature for a book to keep readers hooked. I know I am.