I have noticed that given how much I identify as a rationalist, how much I have in common with the community here, how important I consider it, etc. I have surprisingly little instant in group identification with community members compared to other online communities. There seem to be an aspect of social involvement that LW does bad at. And there is one thing lacking that to me seems the obvious first suspect; the lack of of-topic unstructured chatter.
What I do when I feel that I identify with some continuity online is in fact not usually the thing the community is ABOUT. Instead, it's the things that grow out of the sides; forum games, members art projects, photo share threads, fanworks. I can speculate on why this happens this is so, but it dosn't seem very useful at the moment, I'm not highly confident on any specific theory, and most will probably find it fairly obvious anyway.
LW, however, has no real room for this. Even in the discussion section, things that are not reasonably on topic will be punished with negative karma. Now, this is obviously needed, but one must still recognize there IS a prise to being so structured and focused on a single goal when humans naturally tend not to be. Look for third options.
Now, I have a specific solution in mind, but I'm going to hold of on proposing it and see if you come up with something better before I post my idea.
EDIT: My suggestion has now been added in the comments, please check it out.
To compare to another group:
The lack of social network aspects at Wikipedia are often cited as something it needs, but attempts to start stuff like that have frequently crashed and burned. Whenever "we need to do something" springs up, that's a frequent option for "this is something, therefore we must do this." (Current "we need to do something" this is being mooted as the magical answer for: decline in editor numbers.)
That said, there is still a strong community feel ... and it particularly applies when we meet other Wikipedians in person. This carries back over to dealing with Wikipedians we've only met online, and leads to anticipation of future meetups being good experiences. And all this does in fact appear to further writing an encyclopedia, which is the original point.
So for LessWrong, this suggests: more meetups in more places.
This is fine for those who live close enough to actually have meetups. Far from everyone does.
Also, LW is much smaller and agile than wikipedia, and we have a leader figure that can order people around if it turns out that's needed. Several other factors that differ between the situations as well makes me think it's in fact not very relevant at all.