I think what I liked most about this book was the unique way in which Jay Asher blends the two first-person points of view. The first protagonist, Clay Jensen, is listening to the suicide tapes of his friend (and crush) Hannah Baker. His thoughts and experiences are interjected into her narrative about the thirteen reasons why she killed herself. Sometimes, this is a little heavy-handed--like Clay's thoughts are inserted just to remind us that he's there, but in general the voices are masterfully set apart, and I wouldn't have needed the italicized typeface for Hannah to tell them apart.
The other thing I liked was that Hannah didn't have a massive reason (that we know of) for doing what she does. No rape (of her), no molestation by a teacher--nothing more than the goings on of a small town high school. Asher portrays it in a way for people (teens and adults) to see just how many repercussions their actions can have, which I like.
The hardest part for me as a reader, and for Clay, is knowing that Hannah is dead, and no matter what the tapes say, this can't be changed. This makes for a very gripping, if saddening, read.