When I’m writing software or building a web site, I pay a LOT of attention to the user interface design: whether the most important information is front and center, whether the controls are clear and easy to understand, and so on. This is really, really important. Almost as important as whether the application actually functions the way it’s supposed to.
But a lot of user interfaces are crappy. And the consequences of a poor design can range from mild user annoyance and frustration all the way up to, say, a jetliner crash, or a nuclear plant meltdown.