Amanda, over on the Dead White Guys blog, just posted asking about how people rated books. I'm copying and pasting my comment here, because I'm pretty sure everyone rates books differently. Just like the teacher who hands out As on a rare occasion, my 5 star rating is reserved for something special.
In GoodReads, here is how I have it divided:
5 stars - amazing, life-changing, I will buy a copy if I don't own it
4 stars - solid, good book, might even read again someday
3 stars - okay but not my style, or it didn't live up to the hype
2 stars - just not great, nothing special
1 star - surprised I finished it, a disappointment
What isn't really included here is the way I feel about the rare book I give 5 stars to. It is the book I finish and immediately reread the first bit because I don't want it to be over, or will write fan mail to the author, or stop and read parts out loud because I want to hear them, or literally clasp the book to my chest and take breaks because I want to make it last. That, and only that, is a five-star book.