This is so close to what I was looking for!
Feature Request: Sub-tasks The first interface I use to create tasks is always an outliner, that way I can quickly breakdown everything that needs to be done and add details about their implementation. This is close to mindmaps as shown in your demo, but a text based outliner is faster to type, edit, rearrange in. Having Subtasks would allow an approximation of this.
Also, please start charging for it.