I have no grounding in cogsci/popular rationality, but my initial impression of LW was along the lines of "hmm, this seems interesting, but nothing seems that new to me..." I stuck around for a while and eventually found the parts that interested me (hitting rocky ground around the time I reached the /weird/ parts), but for a long while the impression was that this site had too high a rhetoric to actual content ratio, and presented itself as more revolutionary than its content justifies.
My (better at rationality than me) OH had a more extreme first impression of approximately "These people are telling me nothing new, or vaguely new things that aren't actually useful, in a tone that suggests that it's going to change my life. They sound like a bunch of pompous idiots." He also stuck around though, and enjoyed reading the sequences as consolidating his existing ideas into concrete lumps of usefulness.
From these two limited points of evidence, I timidly suggest that although LW is pitched at generic rational folk, and contain lots of good ideas about rationality, the way things are written over-represent the novelty and importance of some of the ideas here, and may actively put off people who have good ideas about philosophy and rationality but treat them as "nothing big".
Another note - jumping straight into the articles helped neither of us, so it's probably a good idea to simplify navigation, as has already been mentioned, and make the "About" page more prominent, since that gives a good idea to someone new as to what actually happens on this site - something that is quite non-obvious.
For my part, when I discovered the LW material (via MoR, as it turns out, though I had independently found "Creating Friendly AI" years earlier and thought it "pretty neat"), I was thrilled to see a community that apparently understood and took seriously what I had long felt were uncommonly known but in truth rather intuitive facts about the human condition, and secretly hoped to find a site full of Nietzschean ubermenschen/frauen/etc. who were kind and witty and charming and effortlessly accomplished and vastly intelligent.
It wasn't quite like that but it's still pretty neat.
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.