Arch Linux

MediaWiki has a subpage feature [1], but on ArchWiki it is disabled in the Main namespace (this is the default behaviour) and enabled on talk pages and user pages (and possibly on others too, we don't know because of  FS#35545  [2]). There is no reason why the Main namespace should behave differently, so the subpage feature should be enabled also therein.
The configuration is described in [3]; of course if  FS#35545  had been implemented, we could provide the necessary patch.

Background:
Wikipedia describes [4], that they originally used subpages, but then switched to the disambiguation system, which probably lead to disabling subpages by default. ArchWiki does not use disambiguations, and in fact makes use of subpages already (see for example [5], but technically they are not subpages). This has been discussed on ArchWiki, see [6].

Effects:
Full description is given in [1], the most visible changes are relative links using the normal double square brackets notation and automatic breadcrumb links appearing at the top of the subpage, linking to each parent page that exists.

[1]: http://www.mediawiki.org/wiki/Help:Subpages
[2]: https://bugs.archlinux.org/task/35545
[3]: http://www.mediawiki.org/wiki/Manual:$wgNamespacesWithSubpages
[4]: http://en.wikipedia.org/wiki/Help:Subpages#History_of_subpages
[5]: https://wiki.archlinux.org/index.php?title=Special%3APrefixIndex&prefix=Beginners%27+Guide&namespace=0
[6]: https://wiki.archlinux.org/index.php/Help_talk:Style#Slashes_in_titles