The user most likely to engage in retributive downvoting are those who engage in hostile debate and subsequently have low karma ratio's themselves (VoiceofRa has 68% favourability). Perhaps you could disable downvoting functionality for those with a karma ratio lower than 80%? Considering that poor quality of contributions is another big factor for low karma ratios this measure would have the added benefit that our most competent users have more power.
The question very likely was: "What charities should I give money to maximize utility?"
For about a bit more than a year I have been wanting a forum very similar to this one. In the process of gradually unsubscribing from the defaults on Reddit I found that even many of the Truesubreddits were not satisfying any longer and not knowing of the existence of LessWrong I put a moderate amount of effort in fruitlessly trying improving those subs. It took joining the Effective Altruïsm to stumble on this site, the point being that there likely is a considerable pool of potential users for this site who I suspect don't know that this site exists either. I urge anyone active in places like truesubreddits to be more vocal about the existence of this site.
This aspect of accessibility touches on the flipside that Facebook would be too accessible to the wrong kind of potential users, with in addition the format itself possibly leading to a decline in quality. Consider for example that moving LW to Facebook would mean friends of members in the group, despite not being suitable for the page at all, would see they are member and might join to see what it is all about. I once joined a couple groups on philosophy and psychology on Facebook and was very much displeased with the quality of the posts.
I do agree with the idea of expanding the subject matter of this site. Even though I felt a great sense of relief and homeliness when I finally discovered this site this poses a partly unnecessary barrier to becoming active. The lack of breadth in issues being discussed not only means that I currently have to do some homework and have been lurking for weeks prior to contributing anything, but also that those active on the site only can only still the hunger of few of their likely many interests. I wonder if a poll has ever been conducted as to what potential subforums or features users would like to see added/ changed? You have written this post addressing multiple problems and suggestions, but it would have been better if a survey were included in this or a subsequent post to see where precisely the problems lie and to what extent the solutions for them bear support.
One person filled in a 0.51, but even corrected for that the mean is 69,76 (at 17 votes). My hypothesis would be that it has to do with the likely disproportionate percentage of users here that have Aspergers, something that is known to be associated with a poor ability to understand faces.