I haven't really been involved in social bookmarking very long, but isn't Digg what spawned the voting-system type site that we see in Reddit and HN? My understanding is that slashdot based on discussion instead of a straight-up voting mechanic.
Slashdot had moderation for comments, but no similar mechanism for stories. Digg had the voting system for stories and for comments.
I don't know the Reddit history that well, but as I read recently in a PG essay, it was originally slated to be a food ordering application on your phone.
I do not know when they pivoted, or when their idea to pivot came, but I _suspect_ that their pivot was at least partly inspired by Digg's early release.