To be honest i think ratings should be out of ten - and the overall rating is the average of all ratings a particular piece has received.

It's simple and reflective - if a piece has been read once and it is rated 10 - then it is an excellent piece of work and will be listed as so until someone else reads it and a lesser rating is given. In which case its place on the ratings will reduce until another rates it again. 

I think those that are rated once with a 10 will bring other readers in to see what the fuss is about - and so they will hopefully rate too - ensuring that that piece will get more rating which are fair and reflective.

It's basic - It's easy to understand as a rater (1 Poor - 5 Moderate - 10 Excellent) and I'm sure it's easier to calculate than the current rating system.