Big news! You can now specify your interests! Based on the suggestion from several users, I set it up so there's no finite list of things you can be interested in ... just type everything you're interested in! (internally it does some really tricky things to make it fast and small, of course, but that's the topic of a future developer article) Go do it now at the edit info page.
Also, interests are now listed on the userinfo page where you can click them and see other users with the same interest. Please.... use this feature, give me feedback, and bug all your friends to use it as well. This is some really cool data. It makes it fun to browse around by interest.
Very very soon you'll be able to search the user directory by interest as well.