iBizRadio is a podcast directory specializing in business oriented shows.
They use a fixed set of categories with no secondary hierarchy. The topics are well suited to their topic niche, business, and they cover a broad range of businesses as well as business management topics. For long lists, such as podcast or podcast item listings, they paginate over multiple pages. For browsing podcasts they could well stand to have subcategories or more precise categorization for some of the categories that have lots of entries.
It's free to add a podcast. Given the extensive form to fill out, they obviously are manually screening applications.
Their terms of service is a very nice and clear description of the service they offer to podcasters, and what a podcaster can expect from them, and identification of the organization offering their service.