"In life, three things are important: the first is to be kind; the second is to be kind; and the third is to be kind." --Gil Grissom, CSI, sort of quoting Henry James.
My student's will begin critiquing their classmates' writings soon, and, I want them to be aware of this one thing: it is more important to tell someone what he/she is doing well than to find a multitude of faults with an essay.
That's not to say that criticism shouldn't point out areas that need work, but students (or anyone, really) who are providing feedback need to remember that it's the WRITING that needs the help, not the WRITER (well, the writer might need help, too, but is that your job?).
With writing, the focus should always be on how to make the writing better--what can be added, what should be removed, how can the writer pull us further into the essay? You don't have to agree with what the writer has written; you just have to help the writer say it the best way. That's all. He/she doesn't need to write the way you do, or consider the same topics, or draw the same conclusions. The writer needs to find the best way to say what needs to be said. Period.
I always caution my students to temper critisicm with praise. Start with the good stuff to make the "negative" stuff easier to take. Don't say "You need to clarify this"; say, instead, "This paragraph would benefit from more detail. What color was her dress? Her hair?" Point out places where the essay would benefit from a bit more effort. Praise fairly and truthfully.
We all benefit from serious, helpful advice. This is one time that the truth needs to be told, but it needs to be told with kindness.