The MediaWiki Display Title extension is pretty cool. It uses a page’s display title in all links to that page. That might not sound like much, but it’s really useful to only have to change the title in one place, and have it show correctly all over the wiki. (This is much the same as Dokuwiki with the useheading configuration variable set to 1).
This is the sort of extension that I really like: it does a small thing, but does it well, and it makes sense as an addition to the core software. It’s not trying to do something completely different and just sit on top of or inside MediaWiki. It’s also not something that everyone would want, and so does belong as an extension and not an addition to core (even though the display title feature is part of core).
The other thing the Display Title extension provides is a parser function for retrieving the display title of any page: {{#getdisplaytitle:A page name}}, so you can use the display title without creating a link.