Your interface decisions for adding subtasks are not ideal in my opinion, for the following reasons:

- It's the only thing you can edit in the "view" screen of the Task
- clicking the plus (it's a link in the Windows app and not in the Mac app) seems to do nothing.
- In the Mac app you have to double-click (???) the "Add a Subtask" Text in order to make it go away and then you enter a task. I'd be surprised if anyone tried this as the first thing intuitively in the history of this feature in doit :-)
- In the Windows app, you only click once, the text stays and goes away when you start typing (the only indication you get that you can enter a task now is the blinking cursor

So my suggestions:

- Make everything - including subtasks - editable when you double-click and get rid of the additional step of clicking an "Edit" button.
- Make it clearly visible when the user is the right place to add a subtask
- Make the Mac and Windows app behave more consistently (and this goes beyond the subtask issue). Many of us use both and often.