The ability to order items (posts and categories). Display could then be sorted by date, ordering, alphabetically, or some other unique way. This would be useful for blogrolls and the display of categories.

Needed to implement: field added to appropriate database tables ('ordering' seems to be the defacto standard), interface to handle ordering via admin interface. Up/down arrows displayed on a list would be an easy UI way to handle this.

Possible problems: moving items around can get sticky. Mambo does a good functional job of this (see /includes/database.php, function updateOrder() for an example).