Task progress / % complete
I know you probably don't want to turn your application into a clone of other project management software, but it would be nice to differentiate between a task that has not been started and one that is 99% complete.
I've found that this feature is not very useful in other PM systems since the completion percentage is almost always arbitrary. Instead, I'd propose one feature (already on the suggestions forum) that would roughly allow this:
> Task time estimates
Since PT already tracks time logged to specific tasks (and there's a clock icon to show it), you could easily calculate time spent/task budget * 100 = status %
This isn't a "measure of completion" per se, but an indication of whether a task is over or under it's time budget... which is more useful, less arbitrary, and more reliable. For instance, if a task is at 120% of budget, but not complete, you know there's a problem and can act to correct it.