@MarkTraceur: Could you add documentation about the mysterious $wgBetaFeaturesWhitelist config variable?
Topic on Extension talk:BetaFeatures
It's documented in operations/mw-config which is the only place it's used, I believe.
@Jdforrester (WMF) it is an extension global & in the extension code so should probably be documented here in the extension docs.
I added some documentation to the page here.