After you get a taste of LW, every other internet forum feels stupid
And why do you think this is so? Are all participants on this forum genetically superior, and they have to prove it by giving a DNA sample before registering the user account? Or could it be because some topics and some norms of the debate attract some kind of people (and the few exceptions are then removed by downvoting)? Any other hypothesis?
If you propose another hypothesis, please don't just say "well, it is because you are (for example) more intelligent or more reasonable" without adding an explanation about how specifically this website succeeds in attracting the intelligent and/or reasonable people, without attracting the other kinds of people, so the newcomers who don't fit the norm are not able to simply outvote the old members and change the nature of the website dramatically. (Especially considering that this is a community blog, not one person's personal blog such as Slate Star Codex.)
And why do you think this is so?
Well, as for me, reading half the sequences change my attitude a lot by simply convicing me to dare to be rational, that it is not socially disapproved at least here. I would not call it norms, as the term "norms" I understand as "do this or else". And it is not the specific techniques in the sequences, but the attitudes. Not trying to be too clever, not showing off, not trying to use arguments as soldiers, not trying to score points, not being tribal, something I always liked but on e.g. Reddit there ...
(I hope that is the least click-baity title ever.)
Political topics elicit lower quality participation, holding the set of participants fixed. This is the thesis of "politics is the mind-killer".
Here's a separate effect: Political topics attract mind-killed participants. This can happen even when the initial participants are not mind-killed by the topic.
Since outreach is important, this could be a good thing. Raise the sanity water line! But the sea of people eager to enter political discussions is vast, and the epistemic problems can run deep. Of course not everyone needs to come perfectly prealigned with community norms, but any community will be limited in how robustly it can handle an influx of participants expecting a different set of norms. If you look at other forums, it seems to take very little overt contemporary political discussion before the whole place is swamped, and politics becomes endemic. As appealing as "LW, but with slightly more contemporary politics" sounds, it's probably not even an option. You have "LW, with politics in every thread", and "LW, with as little politics as we can manage".
That said, most of the problems are avoided by just not saying anything that patterns matches too easily to current political issues. From what I can tell, LW has always had tons of meta-political content, which doesn't seem to cause problems, as well as standard political points presented in unusual ways, and contrarian political opinions that are too marginal to raise concern. Frankly, if you have a "no politics" norm, people will still talk about politics, but to a limited degree. But if you don't even half-heartedly (or even hypocritically) discourage politics, then a open-entry site that accepts general topics will risk spiraling too far in a political direction.
As an aside, I'm not apolitical. Although some people advance a more sweeping dismissal of the importance or utility of political debate, this isn't required to justify restricting politics in certain contexts. The sort of the argument I've sketched (I don't want LW to be swamped by the worse sorts of people who can be attracted to political debate) is enough. There's no hypocrisy in not wanting politics on LW, but accepting political talk (and the warts it entails) elsewhere. Of the top of my head, Yvain is one LW affiliate who now largely writes about more politically charged topics on their own blog (SlateStarCodex), and there are some other progressive blogs in that direction. There are libertarians and right-leaning (reactionary? NRx-lbgt?) connections. I would love a grand unification as much as anyone, (of course, provided we all realize that I've been right all along), but please let's not tell the generals to bring their armies here for the negotiations.