I find it difficult pinpointing how I feel about a book with just 1-5 Stars or even half stars for that matter. I rate on a point system. The number I come up with takes all of the aspects of the book into consideration. I then come up with my OVERALL RATING from my breakdown of ratings. I grade on these aspects:
Theme or Tone
Flow (Writing Style)
Backdrop (World Building)
Not all of these aspects have as much persuasion as some of the others...for example the book cover rating doesn't really play into my final rating at all. Also, originality doesn't get as much consideration as plot, characters or flow does.
As a added feature, I also give a letter grade rating for anyone who prefers that type of rating.