Digg Podcasts is a user rated listing of podcasts done in the digg.com style.
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.