We write metadata so that we can search through it to find datasets we're interested in using for some purpose. I've recently started building a map-centric user-interface for doing this that has highlighted some simple functional requirements that metadata should meet in order to be helpful in accomplishing the task.
What I've done here is essentially listed the conceptual elements that seem neccessary in order to make a functional user-interface for searching. This has nothing to do with metadata syntax, just with metadata concepts.
The Collection content type acts as an arbitrary logical unit (or container) that groups the Document-like Information Object (DLIO) nodes which hold the actual data resource. At this time, Collections fulfill 3 objectives at once. Perhaps, they need to be separated at a later point.
We just finished putting together a Drupal based Document Repository site (http://repository.usgin.org) as a testbed and as an alternative for other Repository systems such as DSpace. The goal was to fulfilled the following requirements in a ca. 2 week development period (200+ man hours):