From a users point of view, the key requirement is content. That means they want to quickly and easily find what they are looking for. Complex animations, esoteric design and quirky navigation may be OK for some but on business sites they provide the user with good reasons for looking elsewhere. For business web sites the only important web statistic is a sale. What this means is that every aspect of design, content and usability is focused specifically at getting sales.
Simplicity means lower development costs, improved ease of use and lower maintenance costs.
Ensure the site works for all common screen resolutions including 640x480. Ensure the site is compliant with all common browsers and browser versions. Do not use frames - especially on the home page. With a framed web site, your best opportunity for marketing and promoting your site is lost because the home page contains nothing but the frameset. Ensure the home page is a static page rather than, for example, an active server page.