Through evolution, the active queues in my work task list are actually P0, P1, and P3.
I keep separate lists for work, home, personal software develoment, novels, songs, game development, etc. They are prioritized on a mode basis -- when I'm at work, I use the work list; at home, the home list, etc.
I would love at some point in the future to be able to state that 'fun', 'challenge related' and 'worth while causes' would be my priority order. However, in the real world (for the time being) my priority listing covers how I will pay for: Security, Warmth & Food. :)
sev-1, sev-2, sev-3. Well, that is for outages not really tasks. We don't assign priority values to tasks.
When I've seen priorities assigned to tasks, the practical end results always boiled down to: "interrupt current work to get it done", "do it in the 'next' batch of work", or "never do it". Even though there were often more than 3 levels defined (and that 3rd level doesn't really qualify as a priority anyway).