As a peer reviewer, you’re tasked with a delicate combination of responsibilities:
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Identifying any problems in the manuscript or study design that need to be addressed or may make the work unsuitable for publication
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If possible, indicating what the authors would need to change in order to qualify for publication
How do you give feedback that is honest, constructive, and meaningful--without being too severe or nit-picking? Here are a few tips to help put your comments in perspective and balance the critical and constructive aspects of peer review.
Honor the authors’ intention. Evaluate the research on its own terms. The authors may not have asked the same research question you would have; they might have structured the study differently from how you would have done it; reading the manuscript might give you ideas for follow-up experiments--and all of that is okay. As long as the methodology and analysis are objectively scientifically valid.
Differentiate between essential and “nice-to-have.” As you write your review, clearly highlight serious potential flaws that could make the study unreliable, as opposed to minor questions and concerns, aesthetic improvements or suggestions for future investigation.
Take a step back. When you’re analyzing scientific work, details are important--so it’s easy to become absorbed in the minutiae. Once you’ve assembled your detailed feedback, take a moment to reread your comments. Do any of the problems stem from the same root cause? Can you spot unifying themes or patterns in the issues you’ve identified?
Question your own objectivity. We all have a unique perspective, with implicit biases and beliefs. As a peer reviewer, think carefully about the preconceived notions you bring with you to the peer review process and question your own assumptions in order to evaluate the work fairly and objectively. | 
