Every so often, I try to figure out which categories I have not found a waymark in. The easiest way I have found to do this is to look at the "Bingo Card" and hover over each blank tile to see what might interest me. It seems there should be an easier way to do this.
A suggestion would be to add a new tab to the User Page or when selecting the "View all Categories" on the right side of the menu, to add a column of how many submission a person has done to each category to include "0" entries.