I had hoped to complete this feature by now, but as Andy points out, it still needs some work (which is why I hid it and gave it a "beta" label). If you'd like to try it out, select "Popular Topics" from the Browse menu (or just add the "Popular Topics" toolbutton to the toolbar above FeedDemon's browser).
I should add that this feature will probably be useful only to people who subscribe to a lot of feeds since it relies on common links to determine popularity. So if you're not subscribed to feeds which link to the same articles, chances are it won't show you any results. However, if you're a real feed junkie, this feature should really help you discover what everyone is talking about.