Nanotech x-risk would seem to come out of mass-producing weapons that kill survivors of an all out war (which leaves neither side standing), like systems that could replicate in the wild and destroy the niche of primitive humans, really numerous robotic weapons that would hunt down survivors over time, and such like.

What about takeover by an undesirable singleton? Also, if nanotechnology enables AI or uploads, that's an AI risk, but it might still involve unique considerations we don't usually think to talk about. The opportunities to reduce risk here have to be very small to justify LessWrong's ignoring the topic almost entirely, as it seems to me that it has. The site may well have low-hanging conceptual insights to offer that haven't been covered by CRN or Foresight.