Have no separate type of category pages. Categories are displayed by ensuring that all back-links with appropriate relationship are shown in the page.

MediaWiki is confusing on this, in that you can add content to a category page. Wikipedia:Contents/Categories does a reasonable job, but the categories are vast. The P2P Foundation wiki has many categories, some of which are also used for content. Simpler = better. Other wikis have page tags. If there is a specifically different functionality associated with categories or tags, then it would be simpler to name these as different classes of page, but still have the same underlying mechanisms, including for export/import.

Extremely easy: if there is a good backlinking system, then it is removing normal redundant functionality.

In practice, the distinction between content pages and category pages would simply be that category pages show backlinks by default, whereas content pages need not. On my own wiki here I have chosen to show backlinks by default on all pages, but I'm not sure if that is the best decision.

There is one other issue: that on Mediawiki there is, at least in principle, a category tree hierarchy. To duplicate that in a different way we would need to be able to mark pages as category pages, and one of the semantic links would be “is-subcategory-of” or similar. This would have the added advantage of allowing multiple “parentage” in a lattice rather than a strict tree.

Not really an issue: it would be simply a matter of some wikis deprecating some functionality, perhaps to be replaced by page class vocabulary