I think that, given the volume of users on reddit and the volume of content they interact with, any of the various collaborative filtering techniques would work

I think that, given the volume of users on reddit and the volume of content they interact with, any of the various collaborative filtering techniques would work well at this point. You could take it a step further and incorporate more than explicit up/down vote features, such as "clicked", "commented", "saved", etc. Then incorporate some business rules that filter recommendations by subreddits, boost results by time, and now you have a decent recommender. Easier said than done of course.