The comments are an interesting part. Of course you have the complaints that good users/links 'deserve' pagerank.
When Google first demanded that ads have 'nofollow' placed on them & the whole thing got some attention, I was thinking about this and it all seemed unsustainable. The user generated content side of things anyway. The ads thing is of much less consequence. I thought it was a very unimpressive baby-bathwater ratio.
The problem as I see it is this: Most of the 'it isn't a problem' arguments are somewhere around 'if you think it deserves PR, don't nofollow.' The problem is of course the webmaster deciding.
For a site like twitter, spam is more of a problem then not passing their users PR. They have no incentive to moderate their use of nofollow. As is the case in most popular user generated sites. After all, nofollow links don't hurt the source site. Why not use them. Then with CMSs used by companies etc. Why not nofollow all external links? Some 'SEO's might recommend it. No cost.
Problem is that user generated content sites are becoming a big chunk of the internet. Possibly the part with the most important 'recommendations.' How can an SE analyse how all the pages on the internet relate to each other when they cut out all these extremely central sites.
Anyway, I thought this is an example of how SEs are becoming blind to the new areas of the web. Or (maybe more likely?) how they can't actually do what they say they would and pretend a nofollowed link didn't exist.
That brings me to a second point (one for Jill I guess. It's a little awkward bringing up articles/news/external sources in a forum of this kind. Time for HR 2.0 ? Maybe you can have a look at something like this.
* I know some social news sites actually remove the nofollow for submissions that have been positively moderated enough. It surprises me that they bother.
edit: Even worse spelling then normal
Edited by nethy, 07 September 2008 - 11:25 PM.