Forums as comments just for the sake of having readers become writers by starting new discussions is just overkill for self host as there will be very few users (if any!) starting new topics on a forum restricted by a blog topics.

And using external "forum providers" is plain worthless. It's just as @Christina Warren complains: the discussion will take place somwhere else, where there are many users to discuss with, but it's not your content anymore. And is worse than social networks micro-discussion, because people from social network will be somehow compelled to see what the starter read in the first place but from a forum the reader will tend to suffice with the forum discussion or search other discussions for a conclusion/solution.

I think the nested comments along with a comment ordering system putting the most "rich" discussion first is all a blog post needs in term of "fire starting" the comments. Of course there has to be some great content at first! The thing is I didn't see such a ordering system so far (maybe in a beautiful day with two mornings I'll write one...)

What do you think, should a "display comments with the [most replied] at the top of the page" option make sense?