I encourage anybody who's spent any time developing for MediaWiki to go and read https://www.mediawiki.org/wiki/MediaWiki_architecture_document/text -- it's thorough! There are some FIXMEs in there where Guillaume requests more details about performance, caching, and skins, so if you can add some more info on the talk page this week, that would be great.
And it would be wonderful if someone with some Inkscape ability could put together a diagram or two that would help readers understand the architecture of MediaWiki. (Trevor?) Guillaume mentioned to me that he's looked at all the existing diagrams and none are quite right.
Sumana Harihareswara Volunteer Development Coordinator Wikimedia Foundation