Ability to clearly delineate Sub-Tasks
A sub-task is essentially a child task of the parent. So, I would think it's related item would be the task from which it was derived and should show up in reports as such.
If a task has sub-tasks then fields like start and end date should reflect the earliest start date and latest end dates of all the sub-tasks. And the completion status of the parent task should be controlled by the sub-tasks. i.e. the parent task is only marked complete when all of the sub-tasks are complete. If you allow the user to set the parent task to complete, they should be alerted that some sub-tasks are not complete. You could either disallow the status change or have it change all sub-asks to complete as well.
If a task has several sub-tasks, it would be nice to expand or collapse them in the display. It would be nice to show them indented in the task display.