Tagging, the way information is categorized. Yes, the little boxes of text under each question.
It is amazing that it took about 30 years to invent tagging. We used lists and tables of contents; we used things which are optimized for printed books.
However 30 years is much shorter than the time people needed to realize that printed books can be in smaller format. People can keep books in hands.
I think that the tagging concept is underestimated among core CS guys. All research is focused on natural language processing (top-down approach). But tagging is the first language in which computers and people can both understand well. It is a bottom-up approach that makes computers use natural languages.
