I think you hit nail on the head. It seems to me that LW represent bracketing by rationality - i.e. there's lower limit below which you don't find site interesting, there is the range where you see it as rationality community, and there's upper limit above which you would see it as self important pompous fools being very wrong on some few topics and not interesting on other topics.
Dangerously wrong, even; the progress in computing technology leads to new cures to diseases, and misguided advocacy of great harm of such progress, done by people with no understanding of the limitations of computational processes in general (let alone 'intelligent' processes) is not unlike the anti-vaccination campaigning by people with no solid background in biochemistry. Donating for vaccine safety research performed by someone without solid background in biochemistry, is not only stupid, it will kill people. The computer science is no different now, that it is used for biochemical research. No honest moral individual can go ahead and speak of great harms of medically relevant technologies without first obtaining a very very solid background with solid understanding of the boring fundamentals, and with independent testing of oneself - to avoid self delusion - by doing something competitive in the field. Especially so when those concerns are not shared by the more educated or knowledgeable or accomplished individuals. The only way it could be honest is if one is to honestly believe oneself to be a lot, lot, lot smarter than the smartest people on Earth, and one can't honestly believe such a thing without either accomplishing something impressive that great number of smartest people failed to accomplish, or being a fool.
Are you aware of another online community where people more rational than LWers gather? If not, any ideas about how to create such a community?
Also, if someone was worried about the possibility of a bad singularity, but didn't think that supporting SIAI was a good way to address that concern, what should they do instead?
I've spent so much time in the cogsci literature that I know the LW approach to rationality is basically the mainstream cogsci approach to rationality (plus some extra stuff about, e.g., language), but... do other people not know this? Do people one step removed from LessWrong — say, in the 'atheist' and 'skeptic' communities — not know this? If this is causing credibility problems in our broader community, it'd be relatively easy to show people that Less Wrong is not, in fact, a "fringe" approach to rationality.
For example, here's Oaksford & Chater in the second chapter to the (excellent) new Oxford Handbook of Thinking and Reasoning, the one on normative systems of rationality:
Is it meaningful to attempt to develop a general theory of rationality at all? We might tentatively suggest that it is a prima facie sign of irrationality to believe in alien abduction, or to will a sports team to win in order to increase their chance of victory. But these views or actions might be entirely rational, given suitably nonstandard background beliefs about other alien activity and the general efficacy of psychic powers. Irrationality may, though, be ascribed if there is a clash between a particular belief or behavior and such background assumptions. Thus, a thorough-going physicalist may, perhaps, be accused of irrationality if she simultaneously believes in psychic powers. A theory of rationality cannot, therefore, be viewed as clarifying either what people should believe or how people should act—but it can determine whether beliefs and behaviors are compatible. Similarly, a theory of rational choice cannot determine whether it is rational to smoke or to exercise daily; but it might clarify whether a particular choice is compatible with other beliefs and choices.
From this viewpoint, normative theories can be viewed as clarifying conditions of consistency… Logic can be viewed as studying the notion of consistency over beliefs. Probability… studies consistency over degrees of belief. Rational choice theory studies the consistency of beliefs and values with choices.
They go on to clarify that by probability they mean Bayesian probability theory, and by rational choice theory they mean Bayesian decision theory. You'll get the same account in the textbooks on the cogsci of rationality, e.g. Thinking and Deciding or Rational Choice in an Uncertain World.