Tags are very efficient ways of allocating attention in the face of informational overabundance. It takes very little time to bookmark and tag a resource. Because users are the first ones to benefit from classifying the resources that interest them, there is a very high motivation to tag. Thus, what people are doing in reviewing tags is capitalizing on attention allocated by others, specially on aggregated attention (what happens when large groups of people allocate attention to the same tag or resource, as seen in the 'Most Popular' tag or resource feeds in a DCS).The FreshTags system that is under development here takes this "reviewing" of tags a stage further, of course, by allowing readers to carry (or pass) tags between sites, customising the presentation of content based on previously expressed interests.
If you're new to tagging, or want to explore the whole picture, visit Ideant & read the whole essay. Highly recommended!!