There are just two simple things that separate UX design from graphic, web or other sorts of design.
You have to be able to see the world through your users' eyes. You know your users well. You regularly walk a few miles in their shoes.
This is not about dreaming up imaginary users. It's about building empathy for real, actual individuals using observation, talking and plain old being a human. And then you care. You care about what they care about. You care about their response to your designs to the exclusion of concerns about peer or industry recognition.
Your aim isn't to design a web page: it's to design their thought sequence and emotional responses.
What's cool is irrelevant: what delights and moves your user is everything.
UX design moves the needle. That's how you know that it's having a positive effect on the users - and therefore on the bottom line.
Before you start designing, you define the metric you're trying to move and you form hypotheses about what's going to make it move. Your design comes from the hypothesis. Then you test your design and learn from the results. Then rinse and repeat.
This way, it's your users who choose the winning design, not the highest paid or loudest person in your office.
What do you think?
Have I missed something important? Do you agree or disagree? Design of any sort can be tough to define – improve on my definition in the comments!