(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.

I recently used an automatic tracker to learn how I was spending my time online. I learned that my perceptions were systemically biased: I spend less time than I thought on purely non-productive sites, and far more time on sites that are quasi-productive.

For example, I felt that I was spending too much time reading the news, but I learned that I spend hardly time doing so. I didn't feel that I was spending much time reading Hacker News, but I was spending a huge amount of time there!

Is this a specific case of a more general error?

A general framing: "Paying too much attention to the grouping whose items have the most extreme quality, when the value of focusing on this grouping is eclipsed by the value of focusing on a larger grouping of less extreme items".

So in this case, once I had formed the desire to be more productive, I overestimated how much potential productive time I could gain by focusing on those sites that I felt were maximally non-productive, and underestimated the potential of focusing on marginally more productive sites.

In pseudo-technical terms: We think about items in groups. But then we think of the total value of a group as being closer to average_value than to average_value * size_of_group.

This falls under the category of Extension Neglect, which includes errors caused by ignoring the size of a set. Other patterns in this category are:

• Base rate neglect: Inferring the category of an item as if all categories were the same size.
• The peak-end rule: Giving the value of the ordered group as a function of max_value and end_value.
• Not knowing how set size interacts with randomness.

For the error given above, some specific examples might be:

• Health: Focusing too much on eating desert at your favorite restaurant; and not enough on eating pizza three times a week.
• Love: Fights and romantic moments; daily interaction.
• Stress: Public speaking; commuting
• Ethics: Improbable dilemmas; reducing suffering (or doing anything externally visible)
• Crime: Serial killers; domestic violence