I'm harsh on Goodreads but my metrics are:

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5 stars: perfect book, even if I didn't love everything about it if I feel the creative decisions were justified I give it full marks (recent example: Young Hugo)

4 stars: excellent book, any criticisms I have need to be more tangible than simple creative differences, such as trimming the length or redundant word choice (recent example: Babel)

3 stars: average book with good content but was a slog at some stretch. Books that are enjoyable but I find lacking in creativity or other elements go here (recent example: Our Wives Under the Sea)

2 stars: bad book saved by some element of creativity or interesting plot. Usually books that are poorly written but have a good plot go here (recent example: If We Were Villains)

1 star: flat out awful book with few if any redeeming qualities. The longer the book the less forgiving I am of strong segments that would bring it up to a 2 (recent example: The Invisible Life of Addie LaRue)

Perhaps unpopular opinion, but I think DNF's should be their own category of scoring. There are so many novels that fluctuate in scores based on whether or not they stick the landing, so I don't think it's fair for someone to judge a book without the full picture.