Derivative?

In an assessment of my work for a competition to design a sculpture for the entrance of Derriford Hospital,  John Grayson (our course leader) listed some positives and then balanced them against what I assume was meant to be a negative, namely:  "The design concepts were rather derivative in nature."

I emailed John for clarification and got a one liner back, saying: "derivative in terms of source e.g. 'Creation of Adam' and in terms of the panel concept."

I'm not complaining but I'm puzzled by the use of the word "derivative" and it's negative connotation on a number of counts:

• What's wrong with being derivative?
• Isn't everything derivative anyhow?
• What's the difference between being derivative and including references to masterpieces in your work?   
• What proportion of viewers would "get" the references anyhow?