One downside was that because many people didn't hear the 'temporary' part, the longer we kept it running the more confusion was created about what the permanent policy is. Something like "most but not all politics is off-topic, except for this week when it's temporarily all off-topic" turned out to be too complicated.
Another is that the political flamewarring didn't diminish, but merely shifted to arguing about site policy and the experiment. Indeed, it probably ticked up rather than down.
Another is that many good stories that were clearly on-topic for HN but also had political aspects were excluded, making the front page worse not better.
The bottom line is we can't run an experiment like this without changing the site itself in unintended ways. HN is a complex system that way. 'Do no harm' is at the top of our list, and we learned that there's a big leap in potential harm when you go from a 1-2 day timeframe to a week. So I'm sure we were right to end the experiment early; my internal "are things ok" geiger counter was bleeping like crazy at that point.