The most important aspect of WOA is application neutral interfaces (or as some prefer application generic interfaces). Since interfaces are constituted by their IFaPs (Identifiers, Formats, and Protocols), this means that all three aspects must be made "as generic as possible, but not too generic" (playing off Einstein's quote regarding simplicity).
We all know about generic interface operations, eg HTTP's GET, PUT, POST, DELETE, et al. And we are beginning to see more examples of generic formats (or generic schemas if you prefer). For example, ATOM and derivatives like GData.
But its harder to find compelling examples of generic identifiers that are web-oriented, ie, defined as URLs. Well, I just found at great example at the BBC!
See slides 35 and on.
The basic idea is that MusicBrainz uses a GUID-based set of identifiers to uniquely identify artists, albums, and songs, like this URL for Led Zeppelin. So instead of inventing YAUN (Yet Another URL Namespace), BBC wisely decided to reuse MusicBrainz -- as have others (last.fm).
While I'm not sure I like the machine-generated, non-human friendly GUID approach, I do love the idea of id sharing across organizations. Such sharing is the essence of the network effect.