So, any directory of podcasts where Diggnation is the top choice is obviously questionable. To my eye Diggnation is stupid and obnoxious and a waste of bandwidth. In any case the digg.com style is for the digg.com users to vote for the ones they like best. I suppose digg.com users are going to like Diggnation because of who they are. But this means that you'll find podcasts based on popularity of a group of geeks.
Because podcasts are ranked by popularity, finding unpopular niche podcasts means wading through a bunch of popular podcasts that are in the same overgeneralized genre category but are more popular.
If you are registered on digg.com you can really easily add a podcast.