The icons for subtasks should be different from the icons for tasks. Right now the header task and the subtasks all look the same in task detail view, and that just makes the task detail a little harder to process at a glance.
Since there are different icons for task checkboxes (circles and suare) the subtasks should automatically default to a different icon type from the user selected task checkbox icon. E.g. if the user selected task checkbox is a circle, the subtasks' checkbox should be square, and vs versa.