My comment policy has the primary intention of encouraging genuine and relevant comments which add value to the article while discouraging generic, irrelevant, or spam comments.
In order to encourage genuine comments I have made my blog comments dofollow (with conditions). There is plenty of information available through the search engines regarding nofollow vs. dofollow blogs, and why dofollow can be a good thing. My policy is to publish comments which are relevant to the article in question, add value and are not spammy. When an author has published 10 such comments in total, their author url will change from “nofollow” to “dofollow” (ok, technically there’s no such attribute as dofollow, but it is implied by the removal of the nofollow attribute).
To discourage spam comments I run all comments through a filter and then moderate them before they are published on my blog. Any comments linking to offensive sites or those of questionable ethical standards (in my opinion) such as pills, gambling, loans, link-farms, etc. are very unlikely to be published.
Any comments linking to garbage websites or any sites that have been blacklisted by search engines will not be published. This includes generic comments such as “Great Post” which are assumed to be auto-generated unless they contain something specific to the article. Any borderline comments may be published with the url link removed at my discretion.
In summary, if you post an interesting comment and you are linking to a genuine site of good repute then you will be rewarded with some “Google juice”. If you are posting nonsense or spam or have a link to a site of ill repute then your comment is unlikely to appear on this blog.