Really nice app!!
A suggestion for improvement.
Add possibility to use the GTD (Getting things done) method by David Allan - meaning adding an Inbox where the user puts his or her to-do's.
Later the users can take the time to move the to-do's into different categories or calendars when they have the time.
There are already a couple of to do apps utilizing the GTD method. But it has to be easy and quick to add something into the inbox which none have managed yet.
I am thinking. (let's say I have enabled the GTD method in the settings first)
When I start the app I get a little input field where I can write the a short to do. Nothing else. No time set, calendars or anything. Just a small title or a sentence. Later, when I have time I can put the to do's into the right place with notifications etc.
Perhaps it is possible to run the app in the background and connect the note pop up by clicking two times on the home button.
The GTD method is all about getting the idea, or to do's into the inbox as quick and easy ad possible. And then put it into the calendar with meta data later when you have the time.
It is a very appreciated method and I think you would get a lot of buyers if you managed to do a good GTD application in 2Do.