The least helpful comment to receive from a peer reviewer is "It looks OK to me." We want students to find strengths or positive features in a draft, but we need to encourage them to be as specific as possible, both about strengths and weaknesses.
Other points that you should remind students of as you model how to give effective commentary in peer review:
Always point out strengths as well as elements that need more work.
Try to attend to larger issues first (audience, purpose, organization, detail, etc.). Talk about sentences, word choices, punctuation only late in the peer-review process.
Be specific. Point to particular places in the draft where revision will be helpful.
Don't hesitate to respond as a reader, especially early in the review process, for example,
I got confused here.
I saw your point clearly here.
I was convinced by your example or analogy or argument.
If you disagree with the comments of another peer reviewer, say so. Not all readers react the same ways, and divergent points of view can help writers see options for revising.
Make comments in spirit of helpfulness. Take comments in spirit of helpfulness.