Here, before you, is a list of tips for offering helpful critique and encouragement to your fellow writers that I have gathered over my years of writing courses at the university level. This is the result of a considerable accumulation of both good and bad reviews from professors and classmates. Hopefully these tips will not only help you to write a review that is insightful, critical, and yet non-offensive, but also give you tools to think more critically while editing your own work.
Start with a compliment. Every story/poem/essay has some point of merit to it and if you are a good and careful reader you will be able to find that good quality. Be as specific or general as you have to. If you must, you may resort to merely commenting that the idea behind the work has potential or that you laud the effort and courage of the writer for going to the trouble of typing something and posting it on Fiction Press. You may not skip this tip no matter how bad the story is that you're reviewing. Not only will the compliment ease the sting of your critique for the author but it is good practice for you to think of someone else's work in a positive light. If you are too self-absorbed to look past your personal preferences and analyze the piece as a whole and separate entity worthy of your respect regardless of quality then you have no business writing a review. Practice being respectful.
Record your initial and overall impressions of the piece. While these may seem pointless at first glance, due to their general nature, your nebulous impressions are extremely useful to the author. Any serious writer will have a goal (whether consciously or subconsciously) that they are attempting to accomplish with a given work; possibly a particular emotion is meant to be evoked or a specific atmosphere is attempting to be built. Your general impressions signal "success" or "failure" of this goal.
Comment on the appropriateness (or inappropriateness) of style. Based on your general impressions of the work, formulate in your mind what you perceive the goal of the author to be. Does the style of the piece help or hinder this goal? Take into account the genre and the typical motives of said genre. i.e. Fantasy vs. General Fiction. When writing your review make it clear that you are basing your critique on what you think the author is trying to accomplish.
Grammar and flow of sentence structure. Think about the writing in terms of the style it is written in, not the style you prefer to read. Consider the perspective used and the perceived goal of the author. It is appropriate for a piece written in first person to use contractions and, in some cases, vernacular language (remember "The Adventures of Huckleberry Finn") dependent on the narrative voice the author is attempting to create. However, there is a level of grammar and spelling abuse that cannot be attributed to style and should never be overlooked. It is always a reasonable and pertinent comment to suggest that an author find a beta/editor for their work. It is especially polite to offer to take the job yourself but only beneficial if you follow through on the offer. Don't make empty promises of editing help.
Use of detail. Again, it is important to take into account the style the author is writing in. Do the details connect or disconnect you from the primary action taking place? "More detail!" Is not a useful comment unless you specify the type of detail you feel is lacking. "Too much detail!" Is equally useless without clarifying the point at which the details turned you off from the story. Find a detail or moment in the piece in which you felt detail was used well and point to this as an example of a success that the author should cultivate and continue to practice.
Author strengths and weaknesses. If possible, comment on something you notice the author does well. Some ideas might be: witty dialogue (common), believable emotional responses (rare), use of detail, atmosphere, developed style, tension, pacing of action, believable fight scenes (incredibly rare)... the list goes on. Try to comment on a strength before you point out a weakness in a particular area (see preceding list). For every weakness you reveal in the author's work, attempt to offer a suggestion/solution that will help the author improve and turn this weakness into a strength. Examples include: reading aloud, experimenting with another style, rewriting in a new perspective, etc.
Closing comments. Sum up your impressions once more, if appropriate to do so, and end with a word of encouragement, even one so simple as: Keep writing!
I recognize that not all readers have the time to review as thoroughly as my tips would suggest and I therefore recommend that, if nothing else, you take the first and last tips with you plus one other of your choice. Remember that there is no reason to ever fail to be polite in a review. Also, anyone who covers all these points when reviewing your work has put a lot of effort into a sincere attempt at helping you improve your writing and you should therefore not become angry or take it personally when they have something critical to say. Good luck to all of you and I hope you found this useful!
I intend to eventually add to this guide by posting examples of both good and bad reviews and breaking down the points that define them as such. If you have a review you'd like to submit to me for use as an example, please email me.