We should have had this kind of thing as a standard part of our common user-facing software by now; in all software related to reading and writing. All implemented in free software, open protocols and standards, in a distributed manner, not tied to one company or organization. In 2012, I should have been able to practically link a paragraph of my comment here to a line on a local PDF file, another paragraph on that file to a particular portion of a web video, and string a group of such links and transclusions together, comment further on them, keep them under version control, and share them with others, without being tied to a single service or application. This stuff should be infrastructure.
And this is what personal computers were supposed to be all about, before the original naive and optimistic vision of enhancing human intelligence and distributed production and sharing gave way to the current situation where personal computers are used mainly to mindlessly consume centrally produced crap. I'm still looking forward to and willing to work towards an updated variant of that vision, but it's going to be an uphill battle.