Just in case you're interested, this is how I rate the books reviewed here, out of ten.
I try to rate these books based on how well they accomplish what they're supposed to be. Just because a book is "fluff" doesn't mean it has a bad rating; in fact a lot of books that many people would consider "fluff" get great ratings! Another part of the rating is how much I enjoyed reading the book. If it kept my attention, drew me in, and was an enjoyable read, that boosts its rating. It gets bonus points if I'm still thinking about it while I'm not actually reading, too.
Some of the rating does come from stuff like characterization, dialogue, style, plot, pacing--the sort of thing you'd include in a paper for your English class. That's not the main part of it, though, unless flaws with that stuff are extremely distracting from the rest of the book.
Basically, the majority of the rating is more how I felt reading the book than how good the book is, if that makes sense.