This is an interesting question! I feel very unstandardized at times so a lot of the books I read wind up as 3 stars as a result. I think my biggest criteria when I am between ratings is the writing quality. Generally:

5 stars: Very exclusive. I only use this rating for my favorite books of all time. Subjectively, I absolutely loved something about it (characters, plot, world, tone). Can’t stop thinking about it. Has to have high quality writing or creative writing.

4 stars: Usually books that I really enjoyed, read through quickly as a result, but maybe not perfect in terms of writing or plot. Minor flaws.

3 stars: Anything in between good and bad. I am glad I read it, but there’s nothing super special about it either. Some good aspects and some parts I didn’t like.

2 stars: has to have both bad writing + really bad story to be in this section.

1 star: unbearable to read or DNF. I am a completionist so I have literally only a couple DNFs before, read them most of the way through, and feel justified in rating the book even if only partially completed.