Several years back, Ruby wrote a guideline for reviewing stories. It was posted in the FAQ in the library itself. I'm reposting it here for your convenience.

How to write a review: Do's and Dont's

Authors are working hard to present their fine works to you (for free !) and they deserve some recognition from you, readers. At least let them know your opinions. The easiest way is to write a review for a story.

A gentle reminder: Please refrain from judging the work based solely on its content. All stories on this site are accompanied with story codes and synopsis. They are there to give readers some advanced information on the type of content they contain. If you have an emotional issue with any particular type of content or gender code, do the author and us a service by not reviewing his or her work unless you can provide a dispassionate review that concentrates on style, writing skill, novelty, etc rather than your personal distaste for its content.
If such reviews are found, they will not be removed but may be ignored when counting toward the final review rating of the story. This does not discourage you giving low rating score, but please focus more on the story itself and provide a non-biased review.

We also encourage you to give more constructive reviews and comments. A simple "Great story" note is ok, but authors and other readers will appreciate it if you can let us hear more. Here is a short reviewing guide provided by Ruby in the forums:

A challenge to all would be / not so frequent reviewers
Review one story a week. That's 4 a month and with that you could make the top ten reviewers list. One a week - you can do it!
(You'll find it becomes a habit whenever you finish a story and you'll do more if you are not thinking about it.)
Your reward - bragging rights, the right to complain that others aren't reviewing, satisfaction knowing that you helped others... whatever motivates you.
Review writing made easy:
Rate the story based on the description given in the numbers.
Those descriptions are very helpful.
One to three sentences will help the author and future readers. So without giving away the plot or story twists (spoilers) fill in some of the sample sentences. Strive for quality of comments instead of quantity.

The plot was _____ (believable, unrealistic, wonderful, well conceived ....)
The visual descriptions were _____
The characters were ___
I liked this story because ____
I thought this story could use a bit of work on ____ (dialog, grammar, ...)
My favorite character(s) was/were _____ because ____
Needed a warning - please add code ____

Or go with your gut - and tell everyone what you felt/thought:
Samples below - I'm sure you can create your own.

Great writing style kept me wanting more long after I was done.
The story had me reaching for my vibe.
This story was so hot I had to grab my partner.
A perfect story for reading to your partner while they are tied and waiting
in anticipation.

There's nothing to it
Just be polite, have fun and do your best.

Imo, I learn by reading others' stories and feedback. I may not agree with what I'm reading, but I'm getting something out of it.