Let me start by saying I read and review for pleasure. I do not get paid (too bad *sigh*) except for value of the books that are sent to me for review. I am willing to invest my time in reading and reviewing because I truly enjoy it and I like to help encourage and promote authors!
I do not have an elaborate list of check marks for grading as I think that would diminish my enjoyment. I try to note things like settings, character development, plot uniqueness, author voice and general "feel" of the books I review. I have started making notes as I read because otherwise all of the thoughts would be in retrospect. I may not comment on all of those aspects in each review but at least those that strike me.
A reasonable and pretty common rating guide is the 1-5 scale. The 1-5 definitions below are compiled from several sites that I review for. The "in between" entries are my own vague degrees of liking based on my comparing the book to others I have read!
5 - very rare, extraordinary read; a TOP read or GOTTA read
4.25 - 4.75 - better than just a 4 and ranging towards a 5.
4 - a keeper; a FANTASTIC read or NEED to read
3.25-3.75 - better than just good but not quite reaching Need or 4.
3 - good read; MEMORABLE GOOD or WANT to read
2.25-2.75 - better than a Could read ranging up to want.
2 - has several problems; Entertaining or COULD read
1 - very poor read; Leave it on the Shelf
I often get offered a selection of books to review so I am likely to pick in the genres I like and from blurbs I like. Even when I am assigned books, the moderator will have an idea as to my preferences so I am usually going to get books I expect to like.
Any book that gets rated 3 or up is a good solid read! Some will just grab me more than others! I expect I will have a lot in the 4-5 range and a few 3s and a few 5s. I will try to say what I did like as well as comment on specifics I did not like.
If I have a book that falls anywhere in the 2-3 range I will try to be even more specific as to what I did not like about it. It may well be that it is a book that was not for me but it might be a great book for someone else! We all have different tastes and this is a matter of subjectives! My goal will always be to encourage an author with positive feedback and constructive criticism if I think appropriate. I will never bash a book as I would rather just say - "that one wasn't for me."
I doubt that I will have any 1 books for two reasons. First: If I get to pick the books I review it is unlikely that I will pick ones I don't expect to enjoy and that I can't find something redeeming in. Second: If a book made it to publication, it indicates that someone out there liked it so it wouldn't be "left on the shelf" for everyone. BUT I will be honest and say if I ever cannot finish a book or had to force myself to finish. That hasn't happened since I have been reviewing but it has happened maybe three times I can recall over the past several years.
So this is my story on ratings and I'm sticking to it... for now anyway! :)