One thing that is sorely lacking in the realm of extensions is a funded effort to ensure extensions are maintained and kept up-to-date with changes in MediaWiki. Lots of potential donors to the WMF could be coaxed into pulling the trigger if they were given assurance that their contributions would serve to keep their own MediaWiki projects running smoothly. There is no coordinated leadership in the maintenance of extensions, and not only can the WMF fulfill that role, the WMF can also use it as an opportunity for friends of the WMF to find a reason to donate.
Already, it appears the WMF's capability to drive MediaWiki development is outstripping the capability of volunteer extension writers to keep up. Here's a specific example of a popular extension that is perpetually lagging behind MediaWiki:
http://www.mediawiki.org/wiki/Extension:TreeAndMenu
"MediaWiki has undergone a lot of changes in recent versions...It takes a lot of work to write code and it's all done for free, so can take a long time to get the extension up to date with the current versions of MediaWiki."
Even though the WMF does not use all of these extensions, it is the cooperation with all interests that has made the WMF what it is today. There are lots of talented coders out there that have either come and gone from MediaWiki coding, or not come at all, I believe due to the lack of personal resources to devote to their pet projects. Even token bits of funding directed toward developers of popular extensions can do a lot to encourage them to find the time to maintain their extensions, or find someone who can.