Raw

As with all Dave Winer technology, don't treat OPML as an XML vocabulary. It's a serialization (which happens to be XML) of a hierarchical data model which has many purposes in (scripted) programs.

All alternatives you give are obvious for XML people like you and me, but make no sense to scripters that abstract XML away as fast as they can (to avoid DOM hell).

Data formats optimized for scripters don't work for "xml-ers". So there's RSS vs. Atom, XML-RPC vs. Rest and OPML vs., well, XML. It's an unsolvable problem. Let's move on.