[mdc]

That's typically the strategy for blogs. Links in a blog post are known good and don't have nofollow. Links in comments have nofollow to deter comment spam. That works for blogs where the site owner is creating the main content and the comments are secondary.

Sites like StackOverflow only have comments from users; there is no original content from the site owners (except inasmuch as they might participate as users). Using the blog model, it does seem reasonable to not use nofollow on content created by users who have already been vetted by your reputation system.

[mikeash]

I agree that it does seem reasonable to be more granular with nofollow. But refraining from doing so and just applying nofollow to all user content hardly seems like abuse, even so.