Thanks so much; these are very helpful.  (In my case, they made it clear
I've been using <role> and <language> incorrectly, d'oh!)  I noticed a
few minor problems:

* The attribute xlink:href is truncated in some places to just "xlink"

* I cannot find the ISO-3166 country codes at the site given in the
guidelines:
    http://www.din.de/gremien/nas/nabd/iso3166ma
They are available, however, at
    http://www.iso.ch/iso/en/prods-services/iso3166ma/index.html

* In the Detailed Description, <identifier> and <dateCaptured> are
missing cross-references to the corresponding MARC fields.

* In the Detailed Description, the Z39.50 holdings schema example under
<extension> is not well-formed, missing "/" from some of the end tags.

--Andy