User annotated or peer enhanced metadata

Extract from http://www.dlib.org/dlib/may06/arko/05arko.html written by the creators of dlese.org outlines “User Annotation Metadata” well:

“Formal, structured annotation metadata provides a mechanism by which digital libraries can provide additional information about library resources above and beyond that which is included in the master resource metadata record. Annotations can be used to capture, organize, and convey information that might otherwise be lost in the ephemera of emails or list servers, such as users' opinions about the usefulness of a resource or suggestions for adapting the resource for use in a classroom. Annotations can codify the professional judgment of third parties, who are neither resource creator nor library builder, for example, by marking resources that are judged to take an advocacy position on controversial issues. Annotations can flag resources that are of interest to a specialized subset of users, thus conveying specialized information that is only of interest to that sub-audience.

Annotations can strengthen a digital library for education in several ways:

Engage the community. Annotations offer a comparatively fast and easy way for library users to contribute to the library, based on their own experience and expertise, without becoming experts in metadata and cataloging.
Capture diffuse and ephemeral information. Annotations capture ephemeral insights and feedback from users that is otherwise lost in transient media like conversation, emails, or list servers. When this information is expressed in structured records, it is preserved in the library and becomes discoverable. In a digital library for education, such insights may contain gems of pedagogical content knowledge [6], i.e. knowledge of how to teach with a given educational resource.
Increase flexibility. The suite of annotations associated with a particular resource can grow and change rapidly without requiring constant, staff-intensive revision of the master resource record. This increases the library's flexibility and responsiveness. There is no limit to the number or variety of possible annotations for a given resource.
Codify the professional judgment of third parties. Annotations provide an organized mechanism for individuals and groups to delineate whether and how resources align with criteria in which that group has specific expertise. For example, a group with expertise in history of science could use the annotation mechanism to flag resources that present now-outdated viewpoints that were valid at some time in history.
Serve specialized sub-audiences. Annotations enable communities of users to highlight resources of interest to specific audiences. Within such an "annotated collection", each annotation can carry specialized information of interest only to that audience. For example, one could imagine a New York City educators' annotated collection within a larger library, in which a general resource about physical effects of glaciers [7] is annotated to indicate where students can observe glacial striations on a field trip to Central Park.
Disseminate outcomes of a review process. The outcomes could be an overall categorical rating of a resource (e.g., 1 to 5), a narrative summary, scores from a set of rubrics, or some other format.

Why haven't annotations been more widely adopted?

While annotations are valuable for the reasons outlined above, they still face hurdles that have slowed their adoption in digital libraries. No widely-accepted framework for annotation metadata (analogous to Dublin Core [8] for resource metadata) or widely-used tools for the creation of annotation metadata have yet emerged, though the Annotea project [9] and others are working to address those issues. There is no standard protocol for registering annotations so that a resource record can "track forward" to locate all of its associated annotation records. Most digital library builders do not yet incorporate annotations into their public interfaces. As a consequence, users haven't become accustomed to seeing annotation information in digital libraries, and haven't begun to think up new, creative uses for annotations.”