one interesting thing is the inverse relationship between time and the usability and quality of the comment systems.
we start with /., which has--after lengthy iteration--a reasonable system of karma and crowd sourced filtering, plus a brilliant comment presentation system (open/close nodes, point threshholds with one-liners visible before google made that cool).
then we get digg, reddit, hn, each one providing less and less features. hn is the worst: you can't close discussion branches, there is no summarizing, the points are not capped. actually, these charges are against all the "accumulating-points" set. still, i noticed a real drop in features when i went from reddit to hn, predominantly in the lack of sub-domains or categorizations (/r/scifi, /r/python, etc).
anyway....very interesting from a social point of view.