I, like Steve, Paul, and the other respondents, favor a broad open policy. It's important to allow and encourage dialogue--which may involve ad hominen attacks (with which I'm quite familiar).

But Kirsten Anderberg raises important issues. The comment sections of articles could be designated as "open" or "closed". On santacruz indymedia, it would be quite possible to open a second new stream critique-ing the article as a new thread for those who felt strongly about it.
The need to start a new thread might discourage irrelevant smear attacks and would separate them from the article itself.