The place where it really gets bitter is the associated tribalism that political issues bring. A variety of research studies have found that people who identify with a given party will be much more favorable to the exact same topic if told that their party supports it vs if they're told the "enemy" party supports it.

What might have made this experiment more sustainable would have been to scope it to "No partisan politics". Some topics, such as education reform are very political but have no strong consensus in either party. For others, such as IP laws, both major parties come down on one side and many techies on the other. Those topics were common on HN for years before the political discussions here really got nasty.

It's the wedge issues (whatever they may be in a given cycle) and culture wars where the site has really gotten acrimonious. If people are talking about an issue, it's fine. When they're fighting with their allies against the "other tribe" it's not.