No idea if this comment will even get seen by anybody, but I felt like giving some anecdotal insight since I have an oddball curiosity in how website communities grow and how webdesign can make that growth positive or negative.
I've never heard of Less Wrong before 20 minutes ago. Found it randomly via a video game website where someone had linked to an article here, took a few moments to look around...
And I almost bounced from the main page. Why? My guess is jargon. There's a lot of it on your main page, and the first link in the "Welcome to Less Wrong" section leads to a wordy Wikipedia page. As someone who doesn't know anything about "cognitive psychology" or what-have-you, there isn't a clear beginning point to understand what is even being talked about beyond the first paragraph of the welcome section.
Obviously that's just my experience, and I might not be representative of the people stopping by here, by there's my two cents.
Oh, and nice use of the reddit software, made it easy to quickly make an account, even though once I'm done posting this i'm probably never coming back to this site :P