Here are the basic guidelines which I use in my workshops for offering meaningful critique on writer's posts:

1. Do comment on the segments of the piece which made the greatest impact on you.

2. Do point out scenes, dialogue or details which are unclear or confusing for you to follow.  Explain why.  Comments should focus on your individual experience in reading the piece.

3. Do not in any way tell a writer that his or her work lacks value or needs a complete overhaul.

4. Do not try to re-write what someone has written by suggesting plot developments, back-story or alternate outcomes

5. Do accord respect to the creative process at work within yourself and others; this requires time and revision.