My quality rating is now visible to me, by clicking on "Learn More" underneath my stars.
I dug up an email from STN containing some detail of their new ranking system. There are two parts to the system – Schedule and Quality, each weighted 50%. They are tracked on a rolling 12 month period. Schedule consists of completions, timely or untimely, shops cancelled by the shopper, and rejections. Quality is kind of self-explanatory – typos, grammar, clarification requests, and the like. The Quality ranking is from 1 to 4, with 4 being the highest. A 90% score equals 5 stars.
Any other questions, I would suggest asking the help desk or scheduler.