Jeff Atwood's brief post on Ted Nelson's Xanadu ~~failed to~~ mention[ed briefly]{style="color:red"} the aspect of Xanadu that I expected to be worked out by now, which is that links were bidirectional in Xanadu. The closest thing the Web has are trackbacks / pingbacks, which are problematic to spam. I gave some thought to trackback techniques and anticipated blogspam in 2002, but I think with OpenID and FOAF, these issues can be overcome. (More here)