@prem_k @simon @pluralistic no follow was certainly abused, but it came from a very real problem. Pagerank gave you a higher score if more sites linked to yours, particularly if they linked with the search word in the text of the link. So how do you get your site to the top of the front page of Google? You have bots that spam every site with a comment system with links to your site. Slashdot was full of this kind of thing, as were most Wordpress blogs. Adding nofollow removed the incentive.