Implied in your so called 'question' is the statement that any online community that you know of (I shouldn't assume you know of 0 other communities, right?), you deemed less rational than lesswrong. I would say lesswrong is substantially less rational than average, i.e. if you pick a community at random, it is typically more rational than lesswrong. You can choose any place better than average - physicsforums, gamedev.net, stackexchange, arstechnica observatory, and so on, those are all more rational than LW. But of course, implied in your question is that you won't accept this answer. The LW is rather interested in AI, and the talk about AI here is significantly less rational than talk of almost any technical topic in almost any community of people with technical interest. You would have to go to some alternative energy forums or ufo or conspiracy theorist place to find a match in terms of irrationality of the discussion of the topics of interest.
You would have no problem what so ever finding and joining a more rational place, if you were looking for one. That is why your 'question' is in fact almost purely rhetorical (or you are looking for a place that is more 'foo' than lesswrong, where you use word 'rationality' in place of 'foo').
physicsforums, gamedev.net, stackexchange, arstechnica observatory, and so on, those are all more rational than LW.
Can you list some specific examples of irrational thinking patterns that occur on LessWrong but not on those communities? The one guess I can make is that they're all technical-sounding, in which case they might exist in the context of a discipline that has lots of well-defined rules and methods for testing success, and so less "bullshit" gets through because it obviously violates the rules of X-technical-discipline. Is this what you mean, or is it something else entirely?
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.