Whatever type of website you're working on, one thing will always be true: you'll need to give some sort of content.
With a sales letter site, where you're pushing a single product, your sole goal will be to persuade the viewer to make that purchase.
If that's the case, killer sales copy is all that's needed.
Content-driven websites, on the other hand, are a very different scenario. The challenge is, and will always be, to give timely and accurate information. At the same time, you'll be increasing the website's size by adding more subject-relevant (and preferably keyword-rich) pages.