UPDATE: check out my current rating system.
In the process of migrating my book library to Calibre I also reviewed my rating process, all my rating in Goodreads and my top 100 shelf.
This is my new logic for rating books:
★★★★★ - Fantastic book. A classic or a future classic. One of my favorite books.
★★★★☆ - Really enjoyed reading this book. Easy recommendation and a very good book.
★★★☆☆ - I liked it and it is a good, above average, book.
★★☆☆☆ - It was ok, but I would probably read another one.
★☆☆☆☆ - Didn’t really like it.