Sound/Visual effect when completing a task :)
If I complete a task and check the box, the task just disappears as if it was nothing, although I might have spent days in sweat and blood trying to get rid of it. I know this sounds silly, but it would be really fun and motivating to at least get a cool sound effect for all the hard work I've done. :)
Checking a box is one of those "YEEAAH!" moments, and I think they are very important for keeping the morale up.
I agree with Partel, a customizable sound would be a great improvement in both the user interface (I added/updated a task/project) and motivation to finish (completing the task/project).
Maybe the silly part is the "gamification" element but I think task managers need some kind of sound element to complement touch.