An addendum, stimulated by dinner with dfranke: under certain assumptions about the growth rate of the community, karma ages without any algorithmic intervention: as the community grown, the number of moderators grows, and so the mean and maximum moderation per post increases. As a result, new post are effectively weighted relative to old posts.

The problems, of course, are that there is some fixed point, and also that news.yc may not fit the growth model that makes the above true for a community like Reddit--and that may be a good thing.