If you have a login here, you might have recently been playing with pudge's beta test of expandable AJAX comments, with all the weirdness and user-interface strangeness that entails.
Generally, it's been good so far, but I just realised one enormous benefit.
It means that by putting all the data on one page, Google and other search engines are going to be extremely easily able to index entire message including comments, which would have previously been lost.
I only realised this today, when a Google search turned up a use.perl journal post based on content in the comments.
On that basis alone (of exposing more content to indexing) I wonder if it is worth looking at this sort of Ajaxification for all sorts of comment systems. Or at least the sane ones anyway.