Someone, I won't name names, sent me the most marvelously helpful critique of my current manuscript. It was encouraging but also didn't pull any punches and mentioned areas in the manuscript which could be improved upon. It has made me much more optimistic that my story will be, in the end—and I think this is the highest praise—a good read.
I think that's the sign of a good critique, that it helps the writer make their story better without making them feel that their story is a huge steaming pile of doodoo.
Creating a good critique isn't easy. I've written my share of critiques and it's darn hard. I think it's one of those things that is both a learnt skill and a dark art.