Categories, so that whole groups of tasks can be separated from each other. (Eg. shopping, travel list, work..)

Tasks that can be ticked off or on, and left there (ticked or unticked). I use the Palm todo-list this way for shopping lists: I basically have a permanent list of items that I will need now and then (milk, soap..) and untick that item the moment I notice that it's missing from the fridge (or whenever I remember it). Unticked items are automatically sorted at one end of the list. When I find that I have time for shopping I just walk around the shop and tick the items going into my basket. Works great.