Magnus-
Currently, anyone can add and delete categories. I suggest that this will be restricted to sysops later, as it will prevent a "category inflation",
Please, let's not have another ability that is restricted to sysops. Wikipedia is supposed to be open.
Anyone can assign any article to any category,
How? I see no information on how to do this.
All in all, I obviously like my [[Category:Foo]] proposal more. I'm not too worried about separating this kind of meta information from articles, IMHO it belongs there. This makes all changes to the categorization - listed in recent changes - diffable - easy to revert It would also be trivial to use the existing links tables for category management under that scheme.
The reason interlanguage links are so annoying is not that they are mixed with articles, but that they are needlessly redundant. I think someone proposed an interesting solution to that problem a few months ago (hint, hint).
Regards,
Erik