An interface can be beautiful, responsive and well built, and still not help anyone. This section moves up a level, from how something works to whether it's worth using. That's the part of the job where a design engineer has to think like the person who owns the product, and like the person stuck using it.
Most of it comes down to respecting people's time and trust. Pick defaults carefully, because most people will never change them. Make the thing they do fifty times a day as fast as it can be. Never lose something they typed. Keep what they're looking at in the URL so they can share it or come back to it. And ship in a way that lets you take a mistake back before anyone notices.
Most people never change a setting. Whatever you pick is what they get.
The first screen a new user sees is usually empty. Make it the best tutorial you have.
Show what most people need. Keep the rest one step away, not gone.
Words are the part of the interface people read most. Treat them like design.
Find the thing people do fifty times a day and remove every step you can.
Nothing breaks trust faster than typing something and watching it disappear.
If people can't share or bookmark what they're looking at, your app is forgetting for them.
Separate deploying code from releasing it, and every launch gets less scary.