I am quite icky about spelling and grammar mistakes in books (which is quite hypocritical because I make mistakes myself too), so my rating will be influenced with a fair amount of errors.
My ratings strongly depend on my feelings before, during and after reading the book. Character development will forever be more important to me than plot development. So, if a book moves me emotionally than it instantly improves the upcoming rating.
Do not get me wrong, I do review books critically. If a book made me feel something, but it has some problems, I will always mention those in my review. Though, it does not necessarily impact the rating.
Sometimes I compare books to each other. If I gave a book three stars and another one four stars, but the three star book was in my opinion better than the four star book, I will reduce the four star book to three stars or vice versa depending on how I feel. I do the same thing with authors I adore. I compare the book with a book I read previously by them and rate them accordingly.
✰ -> I did not enjoy it and there were way too many problems with the book for me.
✰ ✰ -> I did not like it, but the book was not necessarily terrible.
✰ ✰ ✰ -> I liked it, but did not love it.
✰ ✰ ✰ ✰ -> I really liked it, but it was not mindblowingly amazing.
✰ ✰ ✰ ✰ ✰ -> Mind has been blown.