It would be awesome in incorporate the number of subtasks in the timeline cards. This way if you have 5 subtasks, but 3 are done it say 3/5 or something.
I just closed a task with sub-tasks without even being warned that the task has incomplete sub-tasks. One way to avoid closing the tasks with active sub-tasks would be to see the number of completed sub-tasks vs the number of sub-tasks in the List view.