The saying goes that a picture is worth a thousand words, but on the net text is king. Good design and strong graphics get the attention of your visitors and help build the image of your organisation, but for most situations words are essential to convey your message. And remember too that search engines have no artistic appreciation - words are all that they understand.
The first paragraph on any page is the most important, in appealling both to the visitor and to the spiders which crawl the net to rate your site for Google and its ilk. Ideally it should provide an 'executive summary' of the rest of the content of that page.
The net differs from the printed word in that the page can be of any length, so a 'single topic - single page' design is feasible. My view, however, is that information is better conveyed online in small bites, so even for complex ideas I prefer more pages rather fewer.
A lot of work has been done on ideal column widths for the web. The consensus is that around 12 words per line is best suited to the physiology of the human eye, and that both reading speed and retention fall as length increases beyond this limit. This is unfortunate as most computer screens are landscape rather than portrait orientated. A multi-column layout for text is the ideal, but this is extremely difficult to implement in designs where the visitor is free to increase the font size.