Out of idle curiosity I decided to look at what is happening with the number grades attached to our graded reviews. The grade distribution shows a bias towards higher marks. The average grade given to a reviewed book seems to be about 7/10.
I think that strikes a reasonable balance between being not too fluffy and also not too bitchy? Number grades are all a bit arbitrary anyway... but I think they bring out whether the book was basically enjoyable or not. That isn't always clear from the text of the review.