As you move your mouse around the page DHTML (a.k.a. AJAX) features cause a variety of popups. As you click on items in the topic category, a box appears showing a busy icon, and then the new page loads. Curiously the URL does not change.
They appear to be using a single-level hierarchy of topic categories, which seems to be counter to their goal to index all podcasts. Some of the categories have a couple hundred items, and the "Other" category has 2000 items. Clearly they must add new categories to split up some of these long lists.
The URL does not change as you browse the site. This is because every click turns into a DHTML/AJAX request, making the page reload directly in the web browser without refreshing the page. While this means for better responsiveness (perhaps) it means the search engines are prevented from properly indexing the site. If the search engines cannot index the site, then how can the search engine direct users to the site?
At the bottom of every page there is a "Submit Podcast" link.