Magnus-
some further thoughts&comments on the current implementation:
1) I think the "Categories:" link at the top should only show up on pages that actually have categories assigned to them. Otherwise it's slightly confusing to click the link and end up with a list of categories which have no relationship to the current article (and it also shows up somewhat annoyingly on special pages). If you want a generally available "List of categories", that seems to be a job for a special page.
2) I'm not sure the "|" is a good category separator. When I see
Categories: Biology | Medicine | Health
This suggests to me a kind of hierarchy, whereas
Categories: Biology, Medicine, Health
seems to indicate better that it is a list of items that do not necessarily have a relationship to each other.
3) The categories are not currently displayed in the Cologne Blue skin. Since you seem to like it, I thought I'd point that out ;-)
Once we have this set up, we'll have to think long and hard about related policies. What to put on category pages? Should parent categories be linked when child categories are already linked? Should we replace the current Main Page navigation with a category-based one? Should currently hand-maintained pages like "Needing attention", "Votes for deletion", "Fix a stub" be automated using categories? (Maybe we need MetaCategories?) etc. etc. Boy, this is gonna be fun ;-)
Maybe that could be done "on-the-fly", when showing the edit page?
Yes, that might be worth doing. I suggest waiting with the implementation until it becomes a real problem, though. Of course if it's all optional, I have no complaints about having it implemented right away. :-)
Regards,
Erik