I believe quality is determined by the mechanics of good writing and a writer's potential is determined by the content of the written piece. Combine these two and you have the overall appeal of the written work. However, you must also consider another factor, the reader. The reader brings with them a third "unknown" factor. This is what makes it "impossible" to determine with certainty, the standards to qualitatively, but objectively, arrive at similar conclusions regarding the quality of a written work. Each reader will interpret and react to the written word in different ways, (ie. a Holocaust survivor would have a different emotional response to "The Diary of Ann Frank" than one of Hitler's soldier's) this would change their interpretation of how "good" the written work was.