While reviewing books, I have always had a problem with coming up with a good rating system, that I can follow irrespective of the genre, type, size and author of the book. I mostly just follow the Goodreads system.
It goes like this:
1 star: Didn't like it
2 stars: It was okay
3 stars: Liked it
4 stars: Really liked it
5 stars: It's amazing
I don't find it sufficient though. For one, it is very relative. I may give a 5-star rating to 11.22.63 by Stephen King as well as The Importance of Being Earnest by Oscar Wilde; not that the books are in any way comparable or equal. I thought they were both amazing in their own ways; what I don't get is how to convey this "in its own way" through a rating!
Consider the example of a review copy; where I know it's the author's first attempt at getting published. I have certain expectations from the book and when the book fulfills those expectations almost entirely, I give it a 4-star rating; because I do really like it. That doesn't mean it is even close to being as good as Harry Potter and the Half Blood Prince, which I also really liked (and hence gave a 4-star rating!)
Secondly, I think five stars are too few to judge a book by. So I either use half-ratings i.e. 2 1/2, 3 1/2 etc. Or I use ten stars. Either way, the first problem persists.
I have seen people rate individual elements of a books separately. For instance, theme, plot and characters, each with its own separate rating. This is, I think, the most justifiable method; but I don't know how it exactly works. Which different elements would you rate separately and how do you decide the whole rating of the book?
I don't tend to put much weight in the rating a book holds. Until now I haven't come across a widely applicable rating system. Unless you have one. What do you base your ratings on?