Mind-killer is a name given to topics (such as politics) that tend to produce extremely biased discussions.
It may be particularly important for Less Wrong to encourage a rational perspective with respect to these contentious topics. Nevertheless, introducing these topics into an otherwise healthy discussion (for example, to present an analogy) may ruin it, encouraging fallacious modes of thinking.
Politics as a mind killer
Politics is one obvious cause of mind-killer disputes: Political disputes are not limited to standard disagreements about factual matters, nor to disputes of personality or perspective or even faction: they involve matters that people physically fight over in the real world—or at least, matters that are to be enforced by the government's monopoly of violence. Accordingly, political discourse generally involves an adversarial process where careful deliberation is forgone; the focus shifts onto conflict management and on using arguments as soldiers to advance one's side.
We should expect that existing political allegiances of our users will add bias and irrationality to such discussions. Politically-motivated people may also come here to Less Wrong in order to diligently reduce their casualties and add more recruits to their side.
For all of these reasons, Less Wrong tries to avoid particular political disputes. Of course, discussing conflict reduction skills in the abstract is appropriate and encouraged! In fact, it is the best way of preserving our deliberative, rationalist focus in the face of small-scale disputes and conflicts.
In addition, there may well be a balance of tensions between evaporative cooling of beliefs leading to groupthink, and extremely biased color politics. However, this is an underexplored issue.
Another cause of mind-killers is social taboo. Negative connotations are associated with some topics, thus creating a strong bias supported by signaling drives that makes non-negative characterization of these topics appear absurd.