They have an extensive list of topics, but the topic categorization is only one level deep. Of course one level of categories cannot scale to cover the broadness of all topics. Because they are taking on all podcasts they will eventually need a better categorization system.
They rely on a human-driven editing system to screen all podcasts, and perhaps screen all podcast entries. They claim this produces a high quality directory full of high quality podcasts.
They are open to "anyone" with a passion for a given topic to become an editor covering that topic. The payment is "exposure" in that your contact info and link will be prominently displayed in the channel you edit.