Ratings are a very individual and somewhat arbitrary thing. I like to rate books as honestly as I can - basing my judgement solely on my reading experience and not on my expectation of the book or, heavens forbid, on how others perceive the book. As a result - and because I have been exposed to some pretty amazing reads - my ratings are pretty conservative.
Star ratings explained:
0.5 - Abysmal. Hated the book so much I would not even use it as a door stop.
1 - Really disliked the book.
1.5 - Blergh.
2 - I don't know how I feel about this book: I don't like it but there are worse. This is still DNF country.
2.5 - This is nearly a good book, but there is still something missing - like depth, character development, a plot, a point, etc.
3 - I like it.
3.5 - This was really enjoyable.
4 - I really like it a lot.
4.5 - This was so great it made me think about the book for quite some time after finishing.
5 - This book has made a such an impression on me that I would call it a game-changer.