People are not noticing they have a to-do item assigned to them. Often until the weekly digest email, or when it becomes overdue. It is not intuitive to go to the "My Work" section to check for to-do's.
It would be good to have a notifier icon in the navigation menu somewhere. Similar to what Pivotal Tracker does - see the bell icon with 17 notifications in the top menu bar of the attached a screenshot.