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.