TL;DR: Please add feedback to https://www.mediawiki.org/wiki/Suggestions_for_extensions_to_be_integrated
Since MediaWiki 1.20 has been released, a discussion (opened by hexmode) is ongoing on how to make our release notes better so that users understand what improvements there are and why they should update: http://thread.gmane.org/gmane.science.linguistics.wikipedia.technical/65080 (please join if you didn't yet).
Another problem, however, is making the tarball, which is what most sites use, actually useful. After the first in 1.18, no more extensions have been bundled: Wikimedia projects use no less than 140 extension (most of them functioning) but the tarball ships only 7. A problem that many MediaWiki users have is that they don't understand how vital many extension are, at least for any wiki open to the web.
I'd like 1.21 to include some more, so I ask you to join the brainstorming on our good old https://www.mediawiki.org/wiki/Suggestions_for_extensions_to_be_integrated There are already a few suggestions on it; my personal pet peeve is that MediaWiki is perhaps the software with the best localisation existing but very few use it well because we don't ship LocalisationUpdate.
Topic for another discussion would be to solve some problems with the installer/upgrader like "set up cron for TorBlock" (?) or "check the compatibility of extensions"...
Nemo