Writing for the Web is different. In a word -- it is about keywords. Most people want to be found on the web. That requires not just writing for the reader but also for the Search Engines. Finding the keywords that people search for and then writing pages that reflect those words is where sites begin.
Web surfers often have short attention spans, so you have to grab their attention with graphics and great text. In fact Jakob Nielson of Useit.com tells us that they don't read, they scan. If they want to read anything, they print it out but that's another issue.