When it comes to to-do apps, I am EXTREMELY picky. I have to be, since my mind is a bit, well, flighty.
My preferred configuration has evolved over the years. My litmus test used to be the famed Datebk5 from Palm: desktop and handheld intertwining at its finest. Since then, I have gravitated to cloud functionality. I learned that it was best to get software that I have a reasonable chance of finding on other platforms, and also had a web component that allows me to access data from anywhere that has an internet connection.