Creating sub-tasks is a great feature of projects and issues. When you click on the parent task, there is a good listing of issues and their respective sub-tasks. Unfortunately, on the main tasks list, it is very difficult to identify the task hierarchy. Currently, tasks have labels for parent tasks which makes it somewhat traceable, but it can be quite a mess to try and identify the hierarchy from that list.
I recommend adding some sort of tabbing or bulleting system for sub-tasks in the issues page.