I rate a book based on five categories: Characters, Plot, Originality, Writing, and Ending.
Characters: This is a judge of how much I connected with the characters, and how much the characters grew throughout the course of the book or series.
Plot: This is how much I enjoyed the plot and how much it made sense.
Originality: Have I read a book like this before? Were the characters generic? Was the plot another version of Twilight? If it has the same general ideas of another book, how does it differ?
Writing: This is a judge of how much I enjoyed the writing style, not of the skill of the author.
Ending: How satisfying I found the ending. There are different criteria based on series and stand alone books. If it's part of a series was there just enough cliff hanger to make you want more but not pull your hair out? And if it's a standalone can I easily see what's in the future of the characters to gain some closure?
I give each of these a rating from 1 to 10 so that in the end it's something out of fifty points. I find what that would be out of one hundred and give it a letter grade as well. If you want you can think of an A as five stars, a B as four stars, etc.