Mind-killer is a name given to topics (such as politics or Nazism) that tend to produce extremely biased discussions.
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 on conflict management and on using arguments as soldiers to advance one's side.
As a consequence, we may expect that existing political allegiances of our users will add bias 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.
Unfortunately, the Less Wrong blog is devoted to refining the art of human rationality, not conflict reduction, de-escalation or mediation. Successfully mediating a political dispute requires real-world skills and virtues which go far beyond the virtues of rationality. Many of these "political virtues" were identified by Bernard Crick in his work In Defense of Politics.
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.
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.