I don't like reading about heroines that are shown as being flighty or spoiled. Even if they change by the end of the book I've already disliked them at the beginning and it's hard for me to change my opinion of them as bad as that sounds lol
I really enjoy reading about heroines (or heroes for that matter) who are the underdog. I'm rooting for them the whole book and it makes the happily ever after at the end that much better.