I have participated in a lot of critiques and when I post things on craftster I generally ask for "constructive criticism". I rely heavily on peer review to improve on the things I make. This community is incredibly talented and I like to know what you all think about my projects, whether it be a "great job" or a suggestion for improvement.
In my opinion, the most helpful comments are those that offer a solution. For example, I posted this question here in knitting: http://www.craftster.org/forum/index.php?topic=127884.0 And some people gave very helpful tips, like extending the top, adding a thumb part and one very helpful person even gave me directions for a decorative cast on/ cast off! These were all fabulous and very appreciated without being outright negative. You could easily make a comment like this for a project that you don't care for without a giving an empty compliment or sounding negative.
The comments that I find less helpful are more along the lines of "I don't like x" because they are usually about personal taste and I get conflicting responses; (ie. it seems like for everyone that doesn't like the color there is another person who does). The least helpful to me are those that only offer criticism. I know that many of you have said that you want to be told if one of your projects was terrible, but if someone only left a negative comment, I admit that to me, it would be like some saying f*** you.
In response to your question, I really think that criticism should be reserved to those who directly ask for it.