There is a mind shift that needs to happen for a designer to delve into the world of user experience design. Your aesthetic design sense changes because you're thinking not only of the visual design elements but how they work together. You have to care about the overall look and feel but also of the usability of the application and how users will interact with it. UX design often incites many debates about what it means, it's application, the skill sets people should have if they are to label themselves as such, etc. But the point is that user experience design is important and some people find themselves relegated to that position in addition to being a designer, a front-end developer, an interaction designer, etc. Some of us come from a Human Computer Interaction background, a technical development background, a psychology background, a designer background. All of that I think is great because working in technology these days means being able to adapt, acquire and use different skill sets.
For my role as a UX designer and developer, it means that I have a lot of things to think about... How intuitive is it for people to use the product and to navigate it? What does the visual design of the product say to our users? Does the functional design help to achieve the users goals? How well do the interactive pieces fit together from an architectural point of view? Does the design of the interface support and enhance the user workflow and does the application help to support their needs and goals?
Should all of these things be handled by one person? Probably not. Ideally there would be a team. But in some cases, including mine, you work on all aspects at one point or another. The thing I've learned is to try to separate it. I may be in charge of completing the design from start to finish and perhaps even code it but I try to approach them seperately at first. Tackle the workflow, the design, then usability, then the experience... you may cycle back to the workflow and fine tune it again, then critique the experience and the usability and then redesign. Development might happen after the first pass then you redesign it and make changes to the implementation.
It's a challenging world to work in but can open a designer and developers mind in new and unexpected ways. It also helps to bridge the gap between design and development as we try to create experiences that are effective, effortless and enjoyable.