My rating system is similar to yours, OP. I very, very rarely give 1 or 2 stars. If I DNF I don't feel qualified to judge the book, but if I finished a book and really disliked it then my rule is that I have to explain why. I rarely review, but I think it might be useful in extreme cases to explain why it provoked the reaction it did. I think I've made done this once on Goodreads where there was zero character growth, zero plot development and none of the characters were remotely likeable.