I only started lurking recently, but I thought I'd better decloak at this point.
Metadata is (IMHO) a generally good thing, and the reasons have already been given in this thread. The primary benefit is in making individual applications more useful, but longer term there is likely to be huge benefit through exchange of that metadata. The beginnnings of this are already visible with RSS feeds from blogs, news sites and Wikis.
From the immediate point of view of the developer, it makes sense to reuse
existing work - and there has been a *lot* done in this area. There's a reasonably mature standard for describing resources : the Resource Description Framework (RDF) - some refs below. The key part of this framework is the model, and I'd strongly recommend that this was followed in WikiMedia projects. There are toolkits/APIs for practically every language. (A lot of people get hung up on the RDF/XML syntax, which looks pretty painful, but in practice it's the model that's important and that's pretty simple).
This is all aside from how you'd get metadata from the Wiki - there's all the mechanical stuff (e.g. page creation/modification dates), stuff that can be semi-automated (e.g. author) - management of this is pretty straightforward to hook up to an existing system (most of it's probably already there in the DB tables). Then there's stuff that's entirely human-created. I reckon the MediaWiki is in an excellent position to lead the way on how this is done (what syntax etc.).
Cheers, Danny.
RDF Specs : http://www.w3.org/RDF/
RDF One-pager http://dsg.port.ac.uk/projects/uisb/docs/design/rdf_one_pager.pdf
RDF in 500 Words: http://www.dannyayers.com/docs/rdf500.htm
Some nice pdfs: http://www.semaview.com/resources/resources.html
One of the RDF vocabularies that's been getting a lot of attention recently is FOAF( friend-of-a-friend), which (amongst many other things) can allow you to express social relationships: Fred foaf:knows Bert. People are encouraged to add a 'profile' of themselves to their web site: http://www.foaf-project.org/
A couple of PHP APIs -
RAP: http://www.wiwiss.fu-berlin.de/suhl/bizer/rdfapi/
Redland: http://www.redland.opensource.ac.uk/docs/php.html
I've been working on RDF-enabling a Wiki myself, with the help of the Jena (Java) toolkit: http://66.70.191.189/cgi-bin/mt-search.cgi?IncludeBlogs=1&search=stiki
http://www.hpl.hp.com/semweb/jena.htm
Already the ModWiki and Dublin Core vocabularies are finding application in Wikis, but there's a lot more interesting and useful stuff could be done:
http://www.usemod.com/cgi-bin/mb.pl?ModWiki
It looks like 'categories' is a bit of a permathread around here - hopefully the ongoing work on RDF thesauri will help here: