Michael Erard's thoughtful essay on blog comments in the Sunday New York Times Magazine gives a history of how the comment form of discourse evolved from tech-y backwaters to Slashdot/Metafilter to YouTube diarrhea of the keyboard to mainstream newspaper enhancement (not in that precise order). The central question he's asking is -- could this have turned out differently? What if something like Wikis or annotation-software took off instead of comments as a means of hashing out points?
The closest I've come to the annotation experience was that "weird Google Doc" (as Ryder Ripps called it) posted by Paddy Johnson of the PBS "Off Book" show about animated GIFs that we all disagreed with. You could interlineate your specific criticisms by having comments open up when you clicked on highlighted text. Whole dialogs were happening within these balloons appended to particular text passages. It was a lot like blog comments but crawling up in the author's face EVEN MORE.
Was happy to see Facebook mentioned only once, in passing, in Erard's piece but I wonder if Twitter is also an alternative discourse structure to comments. You have @s and favs appended to a "main" tweet but there's really no main tweet at 140 characters. It's a chaos of cross-pollinating discussion rather than a hierarchy such as you have with blogs. That said, it sucks as a place for debate because of all the misunderstandings resulting from too-short snippets, and it is, frankly, depressing 2 read xclent political writers shoehorning complx ideas in2 bad Mprovised steno.