The ability to grade work to do, or emails to respond to, with up to five priority levels – from unimportant through to super urgent perhaps – as some applications allow you to do, are features that probably only appeal to those who are trying to look busy.
One of my favorite posts is Merlin Mann’s Mud Rooms, Red Letters, and Real Priorities. His main point is that adding an assortment of labels, tags, and priorities to your email inbox only serves to give you the illusion of getting work done. Either something is important, or it isn’t. You don’t need five levels of priorities to decide that, and spending time categorizing isn’t real work.