I agree with Sam. If your stuff sucks, you need to know it, whether or not the critiquer can be polite about it. Any negative feedback stings. You get over it or you get out of the business. The hurt feelings fade, but the lessons will stay with you for life.
I agree with Dale. You have to be honest. Not doing so wastes everyone's time. You can be encouraging, you can be tactful, you can do good writing in reaction to writing as well as for its own sake.
A critique involves two people, so each one is unique. My position is that a newcomer who needs help with grammar and punctuation won't benefit much from a lecture on symbolism or atmosphere. On the other hand, someone with a typo or two doesn't need more than a word or two to indicate the oversight, but would appreciate some pointers on character development. In other words, tailor your crit to the recipient's skill level, be nice, but don't be overly nice.
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