[Rarely do I ever allow comments on pages I have AdSense on. I had done this just because all comments were spam, nonsense, self-promotion, etc. Besides, I rely on good traffic, not people looking to “get that link in there.” It never occurred to me that allowing such stuff may be looked upon as “spam,” and get your site or blog labeled as such. I thought I read the google Adsense blog, but missed this gem: Now, how is a Googlebot (or human even) going to know the difference between spammed comments and not? If you have a blog that has AdSense, and you allow links in comments, think again. I always tell people to be aware of all Adsense TOS. Be forewarned. Now’s here’s another bend. If AdSense users can be detected about spam, how about the Googlebot in general? If you suffered any drops in rankings, look for any pages you allow comments on. Spammed comments can come in all shapes and sizes, not just links.]
Anyone who allows users to post any kind of content needs to moderate that content. If not you are likely seeing the Trust signals for your page reduced or zeroed out.
If you are placing comments on non-moderated blogs, with the expectation that you are generating high-quality backlinks, you are only fooling yourself. Even if your comment is highly relevant, and adds value to the page, you are posting on a page that has likely been reduced to zero trusts, which passes zero link juice.