All the reddit subreddit recommenders I've seen produce garbage recommendations. Outside of a handful of popular, general subreddits which everyone already knows about, everything is niche special interest stuff that you need to find on your own.

Subreddit specificity is so messily complex that it would be very difficult to do any recommendations based on your own subscriptions. Without reddit's cooperation in categorization (unlikely) it's probably not going to happen.

As others already said below, the feedback loop was dangerous.

Also it took a lot of resources to calculate and we just didn't have the time to build efficient map/reduce jobs to do it regularly. It was done by hand in Mathmatica.