Yesterday I moaned about poor use of metadata. Today I'd like to point to an example of the way metadata can be used actively. Andy Hayler describes Kalido's Business Information Modeller, which allows you actively to control and when necessary reshape the data warehouse by changing metadata; you can generate a BO universe, or deploy metadata to Cognos BI tools (more to come, apparently). You can (apparently) even undo the metadata changes, and see your warehouse as it would have been; kind of like Oracle's flashback recovery, but for data warehouse structures.
See Kalido's podcast, screenshots and other resources.