I have a meta-answer: I want LW to have a good shot at advancing humanity's understanding. For that it's not enough to be rational, you also have to be at the frontier of an area. As Vaniver puts it, to make progress you need an important unsolved problem "worked out to third order".
So far the only area where LW has succeeded in putting anyone at the frontier and pushing forward is... yeah... decision theory math. So I'm concentrating my effort there, finding and proving new theorems within the concept space discovered by Eliezer and Wei. If another such topic arises, I'll be happy to help there too.
I want LW to have a good shot at advancing humanity's understanding. For that it's not enough to be rational, you also have to be at the frontier of an area.
I disagree on two counts. For one, I doubt that LW is the best place to develop original research (i.e. to publish ideas and rough drafts and get feedback). For that, you want a different kind of forum or 'subscription list' or wiki or RSS feed. Something with fewer people, most of whom are likely to read what you write. Something more focused. And, more important than a subscription list, you ...
Are there any areas of study that you feel are underrepresented here, and would be interesting and useful to lesswrongers?
I feel some topics are getting old (Omega, drama about moderation policy, a newcomer telling us our lack of admiration for his ideas is proof of groupthink, Friendly AI, Cryonics, Epistemic vs. Instrumental Rationality, lamenting how we're a bunch of self-centered nerds, etc. ...), and with a bit of luck, we might have some lurkers that are knowledgeable about interesting areas, and didn't think they could contribute.
Please stick to one topic per comment, so that highly-upvoted topics stand out more clearly.