Subtasks in Drag are really just comments that you can check off. They lack the ability to assign them and have it show up in someone's task list and to set a due date. IMO to be a truly functional Project Management system, subtasks should be treated the same was as "real" tasks in Drag. They should be able to be assigned, given a due date, priority, etc. They should also be able to be moved from one card to another and one Board to another. I love what Drag can do but find the Subtask functionality really limits what we can do.