I have a stronger suggestion: that any branch in the conversation tree (a post and all its replies, recursively) should be able to be marked meta or orthogonal. Comments are supposed to be rewarded with global "comment karma", I suspect, for the contribution they make to the original topic (the same one that the posted article discusses.) Comments that have nothing to do with the article subject (note that the article itself may be a horrible presentation of the subject; it is not the subject as stated by the article, but rather the subject the community extracts from the article that is the focus.)
Anyway, comments (and all replies to those comments) marked orthogonal enough times by the community would have their comment karma removed, and be collapsed and perhaps restyled. To restore them, you could click on a "seperate discussion" action link on the collapsed thread, which would submit the thread as a separate story, to be voted up and down on its own, orthogonal merits (clicking it again once the story had been so submitted would vote said story up.) If that post, then, achieved a certain (low minimum) score, its posts' comment karma would be reinstated.
Anyway, comments (and all replies to those comments) marked orthogonal enough times by the community would have their comment karma removed, and be collapsed and perhaps restyled. To restore them, you could click on a "seperate discussion" action link on the collapsed thread, which would submit the thread as a separate story, to be voted up and down on its own, orthogonal merits (clicking it again once the story had been so submitted would vote said story up.) If that post, then, achieved a certain (low minimum) score, its posts' comment karma would be reinstated.