Transclusion / Template system
Templates and Scribunto power most of the strucured information across Wikimedia (exception: Wikidata), most notably the infoboxes, links to related pages, and notice templates, but also a huge number of other specific functions. They are an incredibly flexible tool which makes it possible for users to create and maintain things that would never be done if each page had to be updated by hand, and hides complexity from less technical users who can easily include them in pages by copying existing examples.
This is not an easy feature to get right, but something which fills their function is essential to attract editors from any existing mediawiki wiki.
MW started off with basic transclusion+parameters, then added various parser functions in a way which turned out to be ugly and inefficient to the point where they were forced to add (limited) lua scripting.
(WIP, will come back to this with more thoughts)