It's an interesting idea, thanks for throwing it out there. Just to
play devil's advocate a little bit, aren't most of the citations and
external links in Wikipedia articles assertions of "aboutness"?
How is
what you are proposing different?
For example, from the English
Wikipedia Article for Friendship you could derive the following RDF
assertion:
<https://en.wikisource.org/wiki/Essays:_First_Series/Friendship>
dcterms:subject <http://www.wikidata.org/entity/Q491> .
I guess answering my own question a bit, perhaps it could be easier
for people to make these assertions as they are reading material on
the web...and that perhaps not all of them belong in the citation or
external links sections of Wikipedia articles? Some articles could get
a bit long and unwieldy. I remember a social bookmarking site called
Faviki that uses Wikipedia as a controlled vocabulary for tagging
content while bookmarking it. Is that similar to what you are thinking
about?