I am going to assert that the fear of unfriendly AI over the threats you mention is a product of the same cognitive bias which makes us more fascinated by evil dictators and fictional dark lords than more mundane villains. The quality of "evil mind" is what really frightens us, not the impersonal swarm of "mindless" nanobots, viruses or locusts. However, since this quality of "mind," which encapsulates such qualities as "consciousness" and "volition," is so poorly understood by science and so totally undemonstrated by our technology, I would further assert that unfriendly AI is pure science fiction which should be far down the list of our concerns compared to more clear and present dangers.
I'm going to assert that it has something to do with who started the blog.
Why does SI/LW focus so much on AI-FOOM disaster, with apparently much less concern for things like
Why, for example, is lukeprog's strategy sequence titled "AI Risk and Opportunity", instead of "The Singularity, Risks and Opportunities"? Doesn't it seem strange to assume that both the risks and opportunities must be AI related, before the analysis even begins? Given our current state of knowledge, I don't see how we can make such conclusions with any confidence even after a thorough analysis.
SI/LW sometimes gives the impression of being a doomsday cult, and it would help if we didn't concentrate so much on a particular doomsday scenario. (Are there any doomsday cults that say "doom is probably coming, we're not sure how but here are some likely possibilities"?)