Don't know if you remember an old Mac app called In Control. It was a project management tool that was built around collapsing outlines. Ultrasoft's Brainforest Deluxe does much of what In Control did.

The one thing missing -- and this is the feature that I miss desperately -- is the ability to schedule tasks by dragging the task into a calendar on the same screen.

You could accomplish this in OmniFocus. If you added a view that kept the left column of tasks (the collapsing outline) and added a calendar view to the right sector, users could simply drag a task to a starting point. Then, tasks could also be viewed by calendar --- and changing dates could be accomplished by dragging instead of entering data.

To me, this is a killer feature. Don't know why it has been forgotten in this kind of management software.