I am a member of the main organization for my field. Not because I am particularly interested in the organization, or the journals I receive, or because it looks good on a resume. No, I pay $60+ dollars each year for the privilege of being a student reviewer, and spending a few days of my time reading submissions and providing detailed feedback. In the Fall, I review submissions for student grants; in the Spring, I review submissions for several awards or presentation slots in the annual organization conference.
Why pay to volunteer my services as a reviewer? Partly because it's a change of pace from my normal graduate student responsibilities, partly because I'm passionate about making an undergraduate degree meaningful and this lets me train some students on writing and science even when I'm not a TA, and partly because it is interesting. None of the papers I get to review are related to my Area, and it's nice to spend time seeing a difference side of the Field.
Which is not to say that I'm thrilled with some of the requirements of reviewing. The review process is fairly well done. I return a scoring sheet with my final yes/no recommendation, and a one-page review with "a brief paragraph summarizing the proposal in your own words and a couple of paragraphs of feedback". The poor saps who got stuck coordinating each thing review our reviews (with the threat of sending them back for revision), and want them to have both positive and negative feedback. Sometimes this is hard. More to the point, I can't let go of training on cultural differences that says this is confusing.
As an undergraduate, I minored in American Sign Language. Our teachers were all Deaf (capitalization denoting a cultural group rather than simple physical difference), and courses including explanations of cultural differences. Among other things, Deaf people do not see it as rude to ask for information like how much money you spent on something (sharing information being very important in small communities) and do not like the "sandwich" method of feedback that is so popular in business these days. Tell someone good-bad-good, and the result is complete confusion on "am I in trouble? did you like what I did or didn't you?".
The review process wants us to say good things as well as bad, all in one page. Constructive criticism I can do: here's something I had a problem with, here's what I would have preferred to see. Trying to put lots of ego-boosting statements about things that seem so obviously okay/well done is just frustrating. How am I supposed to squeeze details about my substantive comments into a single page if I need to come up with something positive about the methods section immediately before saying "it wasn't there"?