Hm. I took a look at a few of my highly voted comments. Some of them have nofollow, some don't. I can't really tell why. For instance, this answer: http://stackoverflow.com/questions/1990464/efficiency-of-pur... has lots of links in it, citing sources, all of which are nofollowed. A few more of my lower voted answers do not have nofollow. But this one does: http://stackoverflow.com/questions/1946426/html-5-is-it-br-b.... As I go through my history, I find that most of my links are nofollowed; even ones that get more than 25 upvotes, have been around for years, and I'm within the top 1% of reputation site-wide: http://stackoverflow.com/questions/5724522/creating-github-r....

So, either the algorithm is really strict and fairly capricious, or it's quite badly broken.