I think a critique is a two-way street.
First off, if you put your work out there, you have to expect a certain response. I figure that if I am asking for a critique, then I should be open to suggested changes.
Likewise, one doing the critique should offer constructive statements. Both positive and negative or just positive (if there are no negatives). They should also be open to the image and try to learn from it themselves.
Ideally then both parties will come away enriched somewhat. The artist will get valuable feedback, and the critic will also have been enriched by viewing the image and THINKING about it (which hones their analytical skills).
One should also be willing to simply accept that they may like an image and someone else may not. When that is the case I don't offer such an image up for critique.