Currently one can check a task without its subtasks being considered.
This would be less of an issue if there was any indication of subtasks, but there isn't.
Now a user might think he completed "Groceries" while really some of the subtasks "Don't forget to bring back empty bottles" is still left unchecked.
Doit gives no warning, notification or is otherwise helpful in making sure I don't forget to do what I told Doit I still need to do.

"forget to do what I told Doit I need to do". This is of course completely unacceptable behaviour from a todo list application. I hope you change this at your earliest convenience and consider taking on a better interface designer.

Also, I really like your todo list application!