Maybe someday I will become assertive enough to make suggestions where they might be seen. Until then...

• Category directories: Visiting, say, /observations/ would show all lists with the category of "observations".
• Favorite lists: A way to mark one's own favorite lists and/or to mark other users' lists as favorites.
• List background color customization: Provide the possibility to choose custom background colors for lists.
• Personal folders: Users could create personal folders (along with the main four) in order to organize their lists better.