Yeah, that’s been bothering me too. There were plenty of hypertext systems back then (AOL/QLink, Prodigy, Compuserve, Delphi); some were even somewhat open (like Gopher). Berners-Lee just decided on a very small subset of SGML to use for formatting and linking documents, and a very simple protocol for retrieving those documents – granted, it took quite a bit of insight to do it in such a smiple way, but it was the openness which was the real killer feature.