Chris Worth has an excellent exposition of why content is a better marketing tool than advertising - says it better than I can.
Of note are the theories:
- for every piece of web-based content you create, ask yourself "could this be done on television?" If it could, your idea's not strong enough.
- Graphic design can attract surfers to your site - once. But only content can keep them coming back. So forget about design; that's something any art director can slap on in a day.
(NB the link is a .pdf, so you'll need Acrobat Reader)