In the world of scientific peer review, I think that there are three kinds of referees: those that help, those than hinder, and those that are, umm, ineffective. Referees that are ineffective do an adequate surface job, looking over papers to make sure that there are no glaring problems and that the manuscript is appropriate for the journal in question, but that's it. Referees that hinder are the annoying ones we all complain about. You know - they're the ones that send in a twelve word review for your groundbreaking submission to Science or Nature after sitting on it for 6 weeks; the review says little except "Meh." and may even indicate that they didn't really read the paper. They're the ones that say work is nice but not really original, with no evidence to back up that statement. They're the ones who sit on papers because they're working on something similar.
Referees that help are the best kind, of course. These are the people who read manuscripts carefully and write reports that end up dramatically improving the paper. They point out better ways to plot the data, or ask for clarification of a point that really does need clarification or improved presentation. They offer constructive criticism. These folks deserve our thanks. They're an important and poorly recognized component of the scientific process.