First of all, you have Jerry Spinelli's inimitable writing style. He manages to infuse a feeling of epic myth into his stories, even when talking about modern day teenagers in Arizona.
Then you have the character of Stargirl, who is so quirky and innocent, so loving and funny, that she shames you into wanting to become a better person.
Then you have the message of the book, which asks you how important love and a sense of self is to you... when you pit it against social expectations.
Everything about this book is as good as it could be.