Mr. Fraser, who is a web designer, sent me this link from a user interface designer on what he learned about giving good design critique from Tim Gunn. Every word of it is valid for writing critique, in my opinion. I especially liked this point:
Offer direction, not prescription. Tim doesn’t often tell the designer how to fix the design (although he will say what specifically isn’t working for him.) But it is up to the designer to come up with a solution (“Make it work!”).
And also this one:
Accept multiple styles. Tim’s style is, in all likelihood, very far away from the aesthetic of most of the designers. But he doesn’t try to impose his style on them, just sharpen their own while still applying some universal principles of good taste and design.
But it's all worth reading. If you're a writer with critique partners or if you ever judge writing contests, check it out.