Jimmy-
all of the requirements you describe would be met nicely by my [[Category:Foo]] scheme. :-) So far nobody has done any work implementing it, and I won't do much coding for the next couple of months, but it seems like a reasonable solution. I would like to add that any category system should support hierarchies, to avoid unnecessary redundancy. E.g. when you have [[Category:US president with a beard]], you don't need to flag the same article with [[Category:US president]], because the former is a child category of the latter.
Regards,
Erik