Hacker News has never been an anything-goes site. Tight moderation, considerate rules, and low tolerance for bullshit have made this a great site to talk about interesting technical topics and ideas. Remember that we all abide by the rules of the site, and that this isn't a magic free speech zone. If you want to talk political topics, the Internet has more than enough outlets.
Political discourse is antithetical to rational, intelligent discussion. This is not an opinion; look only to sites that allow political discourse (Slashdot?), or even our own comments to see how quickly rational discussion can devolve into flaming. One of the major selling points when I introduce HN to other people is the _absence_ of political topics or discussion: leaving the politics out just produces better technical content.
Also, please consider the idea that politics are regional and differ between countries. In Canada, where I'm from, many of the US political topics would never come up; many European countries might feel even more strongly. As a Canadian, I find American political musings and arguments even less relevant and noisy. By contrast, technological topics are always interesting to me - I can appreciate these, and I love that there's this corner of the Internet where I can participate in a reasoned, interesting technical community. Please don't ruin it with politics, especially the polarizing American variant.
I appreciate that the site is willing to take this step, and I sincerely hope it can keep this site useful, interesting and level-headed for the future.
It would be interesting to know how they will judge the results of this experiment when it concludes.
Personally, I don't plan to discuss politics any more or less than when this topic was posted, so I guess I'm not part of the experiment.