Understanding the problem.

Most of the time the users haven't taken enough time to understand their own problems and have trouble articulating them in a way that's meaningful for a product manager or developer. And on the opposite side of that product managers and developers often do not develop sufficient domain experience to understand the problems users are trying to express.

This is why the absolute best software is built by people who are developing for themselves. You know when it's not solving your problem and you fix it because you know nobody else will.