The 'Add Comments' problem
There is one significant imperfection in my effort to give comment readers as much context as possible: the 'Add Comments' link itself is easily accessible without having read any comments. The problem with this is that the people who are leaving comments are exactly the people you most want to have all of the context.
The obvious thing to do is to only put the 'add comment' link on the bottom of the page that has all of the comments, so people have to at least scroll past the existing comments to leave a new one. But in order to make it clear to a reader how to add a comment, I think you need to do two things: you need to always show the comments on the entry's page, and you can never show the full entry on any index pages, because you need to give the reader a reason to click through to the full entry page. As it happens, I don't like either of those requirements.
You could put the existing comments on the page with the 'add comment' form itself, but this has two issues. First, it plants all of the comments in front of people, ready for them to cut and paste and quote from, and on this issue I'm with Joel Spolsky; the less quoting that gets done, the better. Second, you'd pretty much have to make the 'Add Comment' link jump directly to the form, instead of making people scroll past the comments to get to it, so people wouldn't really be getting much exposure to the comments to start with.
(One way to deal with the second issue would be a two-column page, one column with the 'add comment' form and the second column with all of the current comments.)