The mind of the reader - it's almost completely beyond the control of the writer.
I would add a dire warning to all writers involved with schools and workshops to never forget the need to take that subjectivity into account when evaluating criticism of their work. Both the highest praise and the lowest insult may have more to do with the critic than with the writer.
At a well-known workshop, I had my fiction read by two respected writers. They could not have disagreed more about which of my stories were good, which were bad, and which were just nothing special.
Unfortunately, it will always be up to the writer to decide whether the critique is valuable or worthless.