Rating System Contemplation

I am having some trouble coming up with a rating system. I tend to have a hard time giving negative reviews or low ratings. It is only MY opinion so I guess that tends to take away some of the sting. When I think of a committee reviewing my dissertation and possibly not allowing me to have my PhD I tend to feel like hyperventilating. I mean it is only the culmination of many years of work and a multitude of thousands of dollars that have been invested. I imagine a book author feels the same way. I do not want to ever be the cause of others not trying a book because it fell short of my expectations.