Could you consider incorporating the Urgency/Importance Matrix e.g. http://www.mindtools.com/pages/article/newHTE_91.htm instead of just "Priority"?

The problem is that in all tasks apps you end up with long lists which only takes into account Urgency by Due Date. It's not enough.

Priority should = Urgency factor x Importance (= Necessity) factor and Urgency needs to increase as a Due Date approaches. An algorithm.

I suppose that that would be too much of a change for the user base? Perhaps there is another way of integrating this?