I'd like to suggest "not bad" (the six rating, just below "good" which is a seven) be re-named as "Interesting". Most stories I like are sevens or above, but some that aren't that highly appreciated are still stories others may like. I have the option of reviewing them if I want, although a series of reviews that give six ratings might be a misuse of my reviewer credential, intended to steer readers to stories worth reading and give incentive to writers to keep up the good work. I am not sure a six rating is a strong incentive, and some readers have a minimum score before they will read a story, at least one by an author they haven't liked before.
The principle reason to change the "not bad" rating to something a little more positive is that it isn't really descriptive of the readers opinion. Another option is to call it "average" since that is what a six rating means. The problem with "not bad" is that it also means "not good", at least most of the time. I don't think "not bad" is clear.