The most useful feature (to me) in time tracker apps is the ability to let me know if I've been idle, and options to handle that. (Most apps in this space don't implement this feature)
Have a visible idle counter with suggestions of what to work on next (from your queue). Though we've worried this might frustrate anxious people.
Let you convert an idle block to a task (e.g., if you had forgotten to punch in).
Give you an indication of how much money you're leaving on the table by being idle (via your approximate hourly rate).