As far as the nofollow tag goes, Stephan’s right. Google doesn’t explicitly exclude the nofollow tag’s use on different types of links, and even under your definitions, the Digg Voting button isn’t an editorial link anyway, so it shouldn’t be providing link value.

Most people regard Wikipedia as Web2.0 and it seems to do well in search, even for pages that only have internal links.

Links to forms that are not blocked off with robots.txt may be blocked off by Google algorithmically, but why take the chance? That is not only bookmark buttons but your subscription buttons in the sidebar.

When you see people with blogs that have PR5 or PR6 internal pages, whilst their homepage is PR2, you know there is still some benefit in channelling juice effectively around your site.
I know toolbar pagerank isn’t the best indicator, and pagerank isn’t everything.

If you keep the 80:20 rule in mind, if it only takes 5 minutes to switch from using a plugin that has followed links to social bookmark sites to one that has nofollow or uses javascript, in my mind that is an easy and sensible decision.

For bloggers looking to monetize, splitting feeds isn’t a great idea.

In regards to RSS, if your feed is getting syndicated or read by influential bloggers, then the item title will very likely be used as the anchor text in the link they point to you. If it is not keyword optimized, then you’ll miss some of the opportunity.