I actively used Palm task management for 10 years
and I think they implemented it very nicely:

These are the 5 missing features to make the task mgmt.
in N900 as powerful:

1) Due Date
2) Priority
3) Notes Section
4) User defined Categories
5) Sorting and filtering based on Priority, DueDate and Category

In last update Tasks received custom Alarms!!!

The best list of functional points I can imagine for the Tasks/Calendar applications is the PalmOS application DateBk6 (http://www.pimlicosoftware.com/datebk6.htm) from Pimlico Software. The screenshots have gotten a bit gaudier since I first used it :-), but I have never seen anything on a PDA approaching its functionality.

If I had to pick a single thing, I would say that implementing a quick way of taking a task and automatically generating a linked calendar item is most important. In the N900 metaphor, that would be best accomplished by either a drag and drop into a day in the calendar--or via a long-hold action that would than generate a "pick date" dialog.