Performance, interactivity, control of UI flow, control of layout, and user motivation to try to make a site work are all substantially worse than in other media. Fortunately, some things are more easily done on the web. Site authoring is far easier and faster than with software. A web site can be debugged and updated much more easily. Dave Rollert found working in an ad agency after years in UI design interesting and humbling, because people there routinely thought about things he'd always taken for granted.
Companies have business goals for a web site, reasons for doing things.
Customers have goals and expectations of good service. A web site can be a way customers have direct access to the culture of a company, far more directly than print ads. He explained that a well-designed web site does not feel like it is a replica of the company, it feels like it IS the company. Because marketing brochures are put on the web without enough adaptation to the medium. Also, lack of usability testing means that some percentage of ideas for sites will not be received by users in the way creators meant to send them.