It would be very cute and maybe harmless to be able to mark up some articles as being about people or events or whatever. I'm not sure how this would best be implemented, and maybe it shouldn't be implemented, but hear me out. (I think maybe namespaces aren't right for this idea, but then again, maybe so.)
Look at this text -- I'm not suggesting this as actual markup we should adopt, but this is just something to get ideas flowing:
<person>George Washington</person> was the first <concept>President</concept> of the <place>United States of America</place> after the adoption of the <concept>United States Constitution</concept>.
The magic we could do if lots and lots of articles were marked up with this kind of metadata would be really neat.
If we used namespaces, this would be:
[[George Washington]]
[[person:George Washington]]
But I think we should think really hard about the dangers of proliferating namespaces too quickly. One of the things we have to think really hard about is making sure that any transition to more comprensive semantic markup doesn't frighten and confuse people.
--Jimbo