I have three criteria (four for graphic novels) I use to provide ratings at a glance. These criteria may influence each other (for instance, if the writing quality is poor, it will probably influence my willingness to recommend the book to others), but that is not always the case. For more concrete examples, please see this post.
Personal Enjoyment: my reading experience.
●○○○○ I could not finish this book.
●●○○○ This book was at least worth finishing, but altogether unsatisfying.
●●●○○ I have positive feelings about this book and may pick up others from the series/author at some point in the future. Might also be forgettable.
●●●●○ I really enjoyed this book and am looking forward to more from the series/author.
●●●●● I really loved this book; anything else from the series/author is an auto-buy.
Writing Style: how the storytelling aided/impeded the narrative.
●○○○○ Unreadable. (As I rarely get past the first chapter of books that deserve this low of a rating, don’t expect any reviews.)
●●○○○ Poor; the writing was irritating, bland, and/or made reading into a chore
●●●○○ Average; there were some issues (dialogue; description) that I would have expected from a debut author.
●●●●○ Above-average; compelling writing. Usually one persistent problem either with character or plot throughout the book.
●●●●● Stellar; compelling, immersive, beautiful writing.
Recommendability: how likely I am to recommend this book to others.
●○○○○ I could not, in good conscience, recommend this book to others.
●●○○○ I might begrudgingly recommend this book to others; some books may not be my thing, but they’re probably right up someone else’s alley.
●●●○○ I would recommend this to people if I knew their reading preferences.
●●●●○ I would recommend this book to fans of the genre.
●●●●● I would recommend this book to absolutely everyone.
For graphic novels, I also take into account…
Art Style: how the visual medium aided/impeded the storytelling.
●○○○○ The art impeded or complicated my understanding of the story.
●●○○○ The art was more distracting or unnecessarily confusing.
●●●○○ The art was present and relatively benign.
●●●●○ The art was noteworthy, beautiful, and affected/enhanced my understanding of the story.
●●●●● The art was used in an innovative way, expressed something that isn’t easily captured in words alone, and/or changed my understanding of the story. In other words, the art form and storytelling are inseparable.