At today's LODLAM NZ, in Wellington, I co-hosted a vocabulary schema / interoperability session. I kicked off the session with a list of the metadata schema we care about and counts of how many people in the room cared about it. Here are the results:
8 Library of Congress / NACO Name Authority List
7 Māori Subject Headings
6 Library of Congress Subject Headings
4 Getty Thesauri
3 Marsden Research Subject Codes / ANZRSC Codes
3 Iwi Hapū List
2 Australian Pictorial Thesaurus
1 Powerhouse Object Names Thesaurus
This straw poll naturally only reflects on the participants who attended this particular session and counting was somewhat haphazard (people were still coming into the room), but is gives a sample of the scope.
I don't recall whether the heading was "Metadata we care about" or "Vocabularies we care about," but it was something very close to that.