My problem with subtasks in Jira, is that they're a constantly leaky abstraction. While an epic and task are different on multiple levels, but what makes something a subtask vs a task? This seems like a trivial question, and it's frustrating because it should be, but because Jira has encoded subtasks as a very-similar-but-also-very-different object than tasks, and some actions can only be done against tasks and others only subtasks.