When I say 'hard,' I mean resilient to offenses and stress. Authors, in particular, need some thick skin. I see colleagues react poorly - and I myself am to blame for that - for things that we have not much control on.
I believe that if you are not hard enough, you will never make it. Therefore, I always try to give the most productive feedback possible. If you show me something that is incredibly boring and I think it's not going to make it, I'll say so. I always preface my arguments by saying that I'm no fortune teller. And in the end, I might not be that good at understanding what readers want.