Reviewing books is always a challenge. There are times it feels like reviewers throw a dart at the wall to choose how many hearts/stars/flames/whatever to give a book. I've tried to be clear and consistent when doling out hearts for these books - here are my criteria.
Five hearts: This is a book that just gets inside your soul and stays there. The characters are your friends, their problems are your problems, their happiness is your happiness. You find yourself thinking about these "people" as you stare out the window.
Four hearts: This is a book that makes you think about more than the book. Yes, it's fun and entertaining. But you find yourself bringing it up in "intelligent conversation" with friends and colleagues, as evidence of some important point in life.
Three hearts: This is a book that provides wonderful entertainment. While it doesn't have the beyond-the-book implications of a "four heart" book, it is an outstanding way to spend an afternoon.
Two hearts: This is a book that was fine, but not that memorable. It may be flat characters, stilted dialogue, or an inconsistent plot. Worth reading, but not going out of your way to do so.
One heart: Meh. I guess. If there's nothing else on the planet.
Heartless: Avoid at all costs. These books are just truly terrible. I probably won't ever both reviewing these, but may start a "heartless" list as a public service project. :-)