For those of you new to the art of critiquing other people's fiction, a few words of advice:
The value of the critique is only partially to provide feedback for the author; perhaps the most important part is to develop critical skills that can be applied to your own work. Being considerate of other people's feelings about what they have created is not simply a practical application of the Golden Rulethat is, if you aren't cruel about the other person's story, they may not be cruel about yours when your turn comesit is practice in being even-handed in appraising your own work, neither evaluating it too highly nor hating it too much.
A critique should begin by evaluating the intent. The New Criticism thought this was a fallacythe intentional fallacybut unless you understand what the author was trying to do, you will be unable to evaluate how well the work succeeds. Every story has some ideal form at which it is aimingthis might be called the Platonic theory of fictionand the ultimate goal of the critiquer is to discern what the ideal form is and to help the author see it and how to achieve it. Sometimes that is as difficult for the critiquer as the author, but it is an essential part of helping the author and improving the skills and understanding of the critiquer.
The critiquer should try to see what is good about a story before suggesting ways to improve what does not work as well. But this and the preceding does not imply that the critiquer should hold the story to some lesser standard because the author is not a professional writer. Hemingway said that writing a novel is getting into the ring with Mr. Tolstoy, and writing an SF story is getting into the ring with all those writers who get their stories published. The only meaningful success for a story is publication, and all stories should be measured against that standard: What would it take to make the story publishable? Being considerate of the author's feelings is tactful and tactically desirableauthors accept criticism better when they aren't angry or hurt. But the greatest insult one can offer an author is not to take the author's work seriously and hold it to the highest standards.
The author should understand that not everybody reads stories identically. Everyone brings a different set of experiences and expectations to the reading process, which results in a series of readings that impinge tangentially upon the story itselfthis is part of what is known as "reader-response criticism." The author's job is to put the various viewpoints into contextif the spread of opinions is great, the work may need tightening, the expectations it arouses, more carefully consideration. As for the rest, the author should take what is useful and apply it, and store the rest away for possible later consideration. After all, the final decisions about the form and substance of the story are the author's.
In general, critiquers should avoid commenting about mechanical mistakespunctuation, spelling, even diction or sentence structure except when that gets in the way of communication. A general remark ("the mechanics kept pulling me away from the story," for instance) is sufficient. Focus on the general response.
Everybody should comment on every story but be succinct. Be analytical; don't ramble. Try to get to the heart of your reactions, because this is the art that you wish to cultivate for your own workthe ability to figure out quickly what works for you and what doesn't. You may question individual sentences or even paragraphs that raise questions the story doesn't answer, but concentrate your attention most of the overall impression. That overall impression, your understanding of what the author is trying to do and how well the author has accomplished it, ought to be directed toward what would be necessary to make the story publishable.
Finally, remember that the hardest task writers have is to know when they have written well and when they have written badly. Most writers have creative desires and abilities or they would not be writers; but they often aren't good critics, which takes another set of talents entirely. That is what we learn in workshops, as much through critiquing other people's stories as in having our own critiqued.
Goethe's recommendations for critics is a good standard for a workshop:
"Science into Fiction" Spec-Fic Writing Workshops:
Series 1: "The Higgs Boson in This Particular Universe"
Series 2: "Creativity and the Brain"
Series 3: "Writing in (and about) the Age of Artificial Intelligence"
Chris McKitterick's Spec-Fic Writing Workshop & Repeat Offenders Workshop
Kij Johnson & Barbara Webb's SF&F Novel Writing Workshops and Masterclasses