You need to do following things when you critique other people’s writing:
Start with positive comments.
It can be quite devastating when you submit your work, and you receive a 5-page critique on why your manuscript really sucked. And discouraging/devastating a writer is not the purpose of critique. Also the writer needs to know what he/she did right.
Move on to what didn’t work.
It can only work if you’re specific and give suggestions on how to improve the manuscript. It doesn’t help the author if you write down, “It sucks.” It’d be more constructive and helpful if you write down, “I believe this is wrong because of these reasons. I think you can fix it by doing this. . . .”
End with positive comments, etc.
“Keep writing!” or “I think it can work. Good luck!” can’t possibly hurt. It can take away the “nasty” sting of negative critique and perhaps make the writer feel more positive about his/her work.