Being a user-interface designer involves two jobs: designing a comp and then turning this design into code (or as we say here at Phase2, "doing a cut-up.")
Both involve creativity and precision, but while there is some overlap, at a certain point I take off my designer hat to put on my cutter-up hat. It involves a clear switch of mental gears.
Designing a web site in Photoshop is almost like writing music: layers of colors, textures, effects and other stuff create a picture of a site, a lot like layers of instruments create a piece of music. I say "almost" because designing websites does involve some boundaries; the size of the canvas must meet the limitations of people's monitor settings, for example. However, even making a small button in Photoshop - which might involve six or seven layers - is sometimes like creating a miniature concert.
The design process involves a lot of grey area, and this grey area is getting larger with internet speeds increasing. The ingredients - fonts, textures, layout options - are so varied and so available - it is sometimes difficult to know where to start - and also when to stop.
Of course, a client who says, "I want my site to be green and yellow with my dinosaur logo featured prominently" takes away some of these choices. But there are just as many clients who say, "just use your imagination."
In contrast, doing a cut-up involves less freedom, less creativity, and a clearer set of boundaries. The rules of the site have already been defined in the comp - the site will use a red Verdana font and images will have white borders. Not to say that creating a stylesheet doesn't involve creativity. There are numerous ways to do a cut-up. But the cut-up makes what's been decided in the comp - red Verdana font and white borders on images - happen in code. And unlike design, it is very clear when the cutup is finished.
For me, the orderliness of doing a cutup is just as satisfying as the almost unlimited creativity involved in designing a comp.