[_delirium]

I think it's fairly natural for some topics to become third rails, if they attack views people hold strongly, or are seen to be outside the acceptable range of debated opinion. You even occasionally see it at HN: I recall a thread some months ago where someone expressed a leftist, vaguely anarcho-communist viewpoint on something, and got a flurry of replies that weren't really engaging the comment's content in debate on an intellectual level (if you don't like capitalism you shouldn't be typing this on a computer, accusing him of ignoring gulags, etc.). Certain opinions are just outside various communities' "ok to debate" range, which varies by community, and will get you piled on if you cross a particular community's line (trickier these days because web communities are very fluid and stuff criss-crosses between them).