Here's my "key":
0 stars - In all likelihood, a book receiving this rating is poorly written, has a bad storyline, contains false information (in the case of nonfiction works) and just generally has no redeeming qualities. Admittedly, it's highly unlikely any books I read will receive this poor a rating. Likely those books will end up as DNF's.
Again, it's probably poorly written OR it could contain blatant falsehoods while still being able to capture interest.
The story's just not that great. It's decently written but not attention-grabbing. Or, conversely, the story is interesting but the writing brings it down.
A great book, entertaining and easy to follow. Not perfect, likely has some noticeable flaws, but definitely enjoyable.
You can fit the half-stars in how you will, I didn't see any point in adding them to the key above. You get the general idea, I'm sure. :-)
Please keep in mind that these ratings and reviews will obviously be highly subjective. I do welcome any kind of comments or discussion on my reviews, whether or not you agree with me.