Put simply, web design is an art. It must be approached in a way different than other art forms. However, it needs to be intuitively understood in a way that traditional art does not. There are also additional functional constraints that must be followed to create a memorable and useful site.
Most fundamentally, a website must be visually appealing. As such, good web designers must have a sound aesthetic sense and be comfortable with graphic design. How a site looks is greatly responsible for the first impression users will take away, and a good appearance can make the difference between a site that is busy and one that is closed immediately.
Many sites begin life not as HTML, but as graphical mockups drawn in graphic design tools. Good web designers must not only be good artists, but must be able to command these tools to realize their artistic visions, translating designs from the canvas of the mind onto computer screens for feedback and future revision.
Simply looking good isn't enough, however. Rather, a website must also be understood. In essence, it is a bit like painting a picture with a highly specific end goal in mind encouraging a consumer to buy a product, informing someone about an organization or urging a voter to take action. The best visual appearance is useless if the site's content is difficult to find, non-obvious or overshadowed by its appearance.
As such, competent web designers must also learn about user interface design. Which items are likely to immediately capture a reader's attention, and which are not? Can the site be designed such that the most essential elements are immediately conveyed while others fade into the background for later viewing?
User interface design is a complex field, with much research having been done on the subject. Its introduction into the process adds more constraints, as designs must take such principles into account. If the best design has no one area more obvious than another, or distracts overly from the content, then even the most extensive artistic work is little more than a waste of time.
The web is far more than just a single canvas. As web technologies become ubiquitous, how we view a website takes many forms, and is vastly different on large flat screen monitors than it is on handheld mobile phones. Also, how we interact with sites themselves changes and understanding the differences imposed by various platforms can change a clunky, poorly designed site into one that is a joy to use. Web designers must be more than graphic designers. They must understand these differences in order to create the best experiences for everyone.
Finally, some aspects of web design are not even visual. Good web designers must understand issues of website accessibility as to design sites that meet the needs of the world's large and growing disabled population. Such knowledge includes designing sites that can be read by blind or visually impaired users without access to images, avoiding designs that may trigger dyslexic episodes, and taking any number of other steps to promote an inclusive web.
The technological landscape is in constant flux, however, so it isn't merely enough to acquire this knowledge once. A good web designer must stay informed, learning about the constantly changing state of the art and the ever expanding horizon of new possibilities. By developing a sound aesthetic sense, a deep knowledge of user interface principles and by becoming aware of and remaining up-to-date on technological change, anyone can become a top-notch web designer.