Books I enjoyed so much I'm happy to read them again. Books I've only been able to get through once tend to be the books I donate to the charity shops.
Books that help me "see" what the author sees - where the images are clear and the dialogue realistic - as it is this ability that really takes me into the author's world.
I love stories of authors who get their breakthrough after years of hard work. Those stories tend to give me more hope than those whose breakthrough seems to be "instant".
I also like the fact that age is no barrier, whether young or old, to coming up with a cracking story and getting it out there.