One of the core concepts of user experience design - and of graphic design in general - is the idea of "visual hierarchy." It also happens to be one of my favorite aspects of design.
Visual hierarchy is a design theory that simply refers to the visual relationship between elements in a design that conveys a level of importance of each element in relation to the entire design. It's all about providing guidance as the designer to the audience about what parts of the design or message are most important.
Weak visual hierarchies provide little or no guidance about what is important.52 Weeks of UX
Visual hierarchy is one of the things I often criticize first when a design is poor, and it's one of the elements of a design that I tend to spend the most time laboring over when creating a design.
The reason the visual hierarchy element is so intriguing to me as a designer, is simply that when done well, it gives the designer the ability to profoundly affect the experience of the person viewing your design - the receiver of the message.
Using principles of visual hierarchy well allows you to - and honestly forces you to - distill a visual message down into it's most basic components, and then decide which parts of the message are most important to the communicator and to the audience.
The awesome thing to me about the principles of visual hierarchy is that it is one of the most significant aspects of traditional design that translates directly into the discipline of user experience and information architecture. Simply put - if you as a traditional or graphic designer want to become a better web designer, focusing on visual hierarchy is a great place to start.