I am building an internal tool for ordering in our company. A lot of our people are non-tech so I had to make the app really simple. It is insane how the little change in the UI could affect how people use the app. What seems super simple to me is complicated for other people.
This is why I think it is very important to talk to your users. Also observe them while they are using the app. Chances are you will see the differences in how they use the app and the challenges that they encounter.
The goal is for the app to be super simple so that no documentation is needed. It should be intuitive. Kind of like the iPod shuffle, you just press play!