I think this is the same thing I asked for here.

One way to have a go at this would be to use the time estimate field as a proxy for % complete. I realize this is not helpful if you presently use the estimate, but if you didn't, you could then plug in time values that stood in for your percentage. You could then build on that with smartlists that would show you, for example, tasks with 0% done, or tasks over 75%. It's a kludge, but it might do the trick.

I would like a % of the tasks in a list that are completed. This would be great for staying motivated. This is especially important since completed items disappear and you cannot tell quickly how far along you are.