Obscure site structures that either have no logic or are based on the company's org.chart. Lack of navigation support, making it very hard to find things when combined with an obscure structure.
People only visit a bad site once but become loyal users of the good sites.
Provide search if the site has more than 100 pages. Instead of cramming everything about a product or topic into a single, infinite page, use hypertext to structure the content space into a starting page that provides an overview and several secondary pages that each focus on a specific topic. The goal is to allow users to avoid wasting time on those subtopics that don't concern them. Use product photos, but avoid cluttered and bloated product family pages with lots of photos. Instead have a small photo on each of the individual product pages and link the photo to one or more bigger ones that show as much detail as users need.