Open Thread: March 2010, part 2

The Open Thread posted at the be­gin­ning of the month has ex­ceeded 500 com­ments – new Open Thread posts may be made here.

This thread is for the dis­cus­sion of Less Wrong top­ics that have not ap­peared in re­cent posts. If a dis­cus­sion gets un­wieldy, cel­e­brate by turn­ing it into a top-level post.