Peer reviewing is a sensitive process because no body enjoys being told that their work isn't good enough. We must be aware that our peer has put in a lot of effort to complete the assignment, and we cannot just jump in head first and bash their best efforts. To become an effective reviewer, I need to STAY POSITIVE! That means to first find what does work in their writing rather than what doesn't, and then proceed from there. For example, I could say, "I really like what you're trying to say/describe and maybe with a little sentence revision it will be more clear to the reader." instead of saying, "What you wrote doesn't make any sense and your sentences are too unorganized." The attitude I have when I critique something can determine how the peer responds and if they fix it correctly.
Furthermore, I always thought it was only about pointing out the spelling and grammatical errors, but now I understand that I need to look for things like a clear main idea, smooth transitions, descriptive detail, and etc. We are doing this to help one another, so to provide meaningful feedback we also need to make useful suggestions in the peer's work. I could suggest moving a couple of sentences around to be read more clearly, or provide synonyms for a word that would create more meaning, or even suggest how to make a conclusion. I believe that when peers brainstorm together there shouldn't be any reason why an essay or a blog isn't up to the correct standards.