In my experience with doing multiple peer reviews this year, I felt as though editing another persons paper was not only beneficial for them, but also for myself. When editing another persons paper, you are exposed to different writing styles as well as may notice grammar or format mistakes you have made in your own essay and can edit it correctly. I think my peer review skills have improved with each one, as my own writing improves. In my comments, I try to focus on sentence wording, format, how a quote fits in, and the overall flow of the paper. In my end notes, I just summarize my comments and if there is a big issue that seems to be holding the paper down I will discuss it in the note.
linked in drop down menu, my comments are in red