Tod Maffin has a definition of a successful podcast (at least one established enough to possibly be part of his new podcast collective) in this post:
We already have the lineup we’re launching with, but we may be interested in talking with you if you have an established podcast (i.e. more than 2,500 subscribers) and a specific topic. Here’s what you should know: We’re looking for podcasts with high production values (i.e. we’re not interested if you only ramble into a microphone, upload it, and call it a show). You should have good pacing, have a good lineup going into the podcast, and use the time well. Your show should move and it should be informative and entertaining. And it should be of a specific topic that we’re not already covering: cars, teen living, science, women’s health, crafts, etc. So, still fit the bill? Email firstname.lastname@example.org and tell us about your podcast.