> StackOverflow is harming the quality of results in Google because they want people to filter through them for the information.
Well, no, actually. StackOverflow may be shooting itself in the foot. SO cannot dictate the rules of the search game to Google, and Google adjusts its SERPs pretty quickly, especially nowadays, when user behavior gets to have bigger influence on SERP with every ranking algo update. Poor quality content leads to higher bounce rate and lower avg. time on site and visit depth, which will inevitably lead to lower ranking and less traffic for SO. This (and perhaps a thousand of other factors) works for every site, and even more so for heavy traffic content projects like StackOverflow. So don't be afraid, the system will adjust itself.
Overall, I agree with your sentiment: I love SO, but nofollowing the Web looks kind of lame and selfish. After all, doesn't SO have its staff of moderators and admins to fight spammy comments?
Our suspicion is that when we clamped down on the spammers by banning accounts they reported our site for spam that they had created we lost 20% of our traffic - major brands like us can tough it out this could kill smaller sites without the runway to survive this or have "friends" inside the wire at Google.
I have had to help a small company completely rename and start again on a new domain after some one hacked the site and inserted 1000's of pages with links to porn sites.