Almost all avid book readers I have met have their own personal star rating system. Even those that say they "wing it with their mood", have consistencies about what ratings they give books. I spent some time thinking about mine, and this is what resulted. I do not rate books I do not finish.
A book I did not like and would not recommend.
An okay read, but one I probably wouldn't recommend unless it seemed really up your ally.
A solid, enjoyable read and might recommend based on your preferences and tastes.
A great book I would hands down recommend.
A fantastic read with great writing, stirred emotion, and is a must read.
These indicate it was a hard decision on what rating to give it, but it was missing something to go to the next star up.
Typically this means it is an autobiography or memoir.
Or perhaps I opted not to give it a rating.
What does your rating system entail? I'd love to know!