In light of those who choose to bash poetic endeavors, instead of leaving a proper literary critique or review, here are some suggestions on how to be effective and still be nice. You don't have to like it. Just don't trash it. Tell them which part particularly appealed to you and why - how did it make you feel? What did it make you think about? Be insightful, not insulting. Help them to see and understand their own piece in a new light; I guarantee they'll appreciate it. Keep it simple, honest and pure. I send corrections privately, if possible. I don't believe in public humiliation for a typo. I rarely make suggestions for change unless I truly believe it will help the poem become better; then, I am kind in my language and do not ever attempt to tell them how to write; each person has their own individual style. Each of us has our own unique voice - it is that very quality that makes us value our own creativity. No one sees the same subject exactly the same way. Be supportive, above all. Creative people are extremely sensitive and usually quite intelligent, as well. Remember that, act accordingly and expect the same in return. Most will oblige. Above all, be generous - no one likes every single thing they read. Even if you don't like the piece, find something positive to say about it; for example, that their form was original, their symbolism was strong or even that their background was well-chosen. Be thoughtful in your comments and they will not only return the favor, they will do so graciously, with appreciation.
There are infinite dangers involved in even daring to critique another's work. Each writer displays their very soul upon the page with both utter insecurity and great pride. I often say that a poem is a poet's baby. One might as well leave the comment, "What a lovely child. If only you'd move their nose to the other side of their face, then (and only then) they would be beautiful." It is a wise thing to bear this in mind when visiting another's pages; their very lifeblood is spilled across this cyber-parchment with the hopes of garnering insight much more than mere praise. With this in mind, I shall only suggest to you to be careful, not careless with your remarks. There are those who do not take kindly to being told how to write, especially by someone who has not yet written enough to even qualify as a competent reviewer. I have seen comments that resembled crime scenes left upon various sites. I have also seen a lot of bashing go on, which is not what a poet is or should ever be. There are young writers that take such horrid comments too much to heart, deleting their works, leaving the site and perhaps, never writing again. What a tragedy that is; we all need a creative outlet, in order to become human beings. This world needs all the artistic souls it can hold. There are also quite a few very well-established, published writers on different sites who do not seek actual criticism at all; rather, they wish only for insightful commentary. Their work is as complete as any poem ever is. Poets are meant to be creators, not destroyers; anyone who suggests otherwise needs to rethink their stance as a writer, or any other sort of artist. These are just a few suggestions for improvements in critiquing, by one who does take this craft, this art, very seriously.