The most important aspects of excellent, high-quality reviewing.
The Five Commandments
Above all. You must be prepared to be straightforward with the writer you are reviewing, and they either will or will not be prepared to accept that. Without truth there can be no progress.
No one will benefit from criticism that is all harshness and denouncement, nor will anything be learned from unmitigated gushing praise. To temper criticism with kindness is an artform that all who would review must learn.
We are all cut of the same cloth, and are connected by our common passion for the written word and our craft. Every writer you review, no matter their caliber or level of experience, is your peer and thus deserving of the utmost consideration that you yourself would expect to receive.
There must exist a careful equilibrium between the relating to a writer of what you felt was done well in their work, and what you felt could have been improved. Stating only one or the other will not show them where their strengths lie, or help them to realize and address their weaknesses.
Of all the important aspects of reviewing, commitment to completion and detailing in the process is perhaps the most vital. A reviewer must have the attentiveness to not only state why something worked or did not, but also be knowledgeable enough to make viable suggestions with confidence to the writer on what can be done differently. This can only be achieved through hard work, focus, and discipline, but the rewards are tremendous.