Credit:
1. A Tasks API and synchronization
2. Reminders and notifications
3. Recurring tasks
4. Shareable task lists
5. Visual distinction for overdue tasks
A Tasks API and synchronization, which heads the list, would let users spread their task lists across their favorite to-do apps. With just this one update, Google Tasks would be significantly more useful.
Various nags might be handy, too. For instance, we wouldn't mind being reminded from time to time that a task has been languishing, untouched. Nor would it be too emotionally taxing to have some sort of visual distinction for tasks left undone.
Google Tasks has already been useful, and these features could put it over the top, perhaps into indispensible territory.