I like the "Time Machine" feature of going back 1 (to N) year(s) in the past, it brings a lot of insight as to how technologies and trends evolved (for those of us who have only recently joined Hacker News).
This is one area when the functionally can really be improved over what reddit implements. Having a moving window of time to sort within is an amazingly useful feature for sorting out trends or lost articles. The expanded granularity would be very much appreciated.
Also with Reddit being able to trace causality between memes and such would be interesting.