Sorry if you came here wondering if you should nofollow comments or something else on your site or blog, but I'm looking at it more from a crawler's perspective. Though if that is why you're here you have something to add. Basically the nofollow either in meta tags or as a rel attribute of an a tag is a hint to a crawler telling it, in it's most basic terms, not to follow the link. But what does that really mean to me, if I run the crawler? More importantly, how is it actually being used?  On blogs, personal websites and even wikipedia the nofollow policy is pretty clear and transparently aimed at preventing spam - and it works. But how are they applied? Well, not even the big search engines treat them consistently: Google completely blanks them, Yahoo indexes them but adds no juice, and Ask doesn't even support them [Granted this article may be somewhat out of date].  One interesting thing that came out of the answers though, was the one from Google: 
On a related note, though, and echoing Matt’s earlier sentiments… we hope and expect that more and more sites — including Wikipedia — will adopt a less-absolute approach to no-follow… expiring no-follows, not applying no-follows to trusted contributors, and so on.
So within even Google, with the strictest application of a nofollow policy, there is certainly a strong argument and use case for treating this as a hint as opposed to a policy. I'm not even sure why Google needs wikipedia to make a policy change, everything they hope for could be implemented at their end.  A link on wikipeida, if it's been there long enough, probably deserves some juice. And that same logic applies to any site, even your blog - nofollow is a hint to a search engine. It's there to deter spam, but if a link sticks around then by not removing it, to a point, the site is endorsing it.  For our application we're working from a slightly different angle - there aren't a lot of geotagged urls and there isn't much span so at least initially we want as many as we can get. So we may not index or pass juice on to the site, at least initially, but we do want to follow the link. 
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