1. Not knowing, before you design your webpage, why anyone (besides you) would feel compelled to visit it.
2. Not knowing, before you design your webpage, why anyone in their right mind would return to your webpage on purpose.
3. Lack of focus. Not knowing one, single thing that visitors come to your page to have satisfied. (If you actually have more than one way of satisfying visitors, then, unless they are *intimately* related, put them on separate pages. Better yet, separate sites. Think infomercial rather than search engine. Think television show rather than channel surfing.)
4. Not knowing --and, therefore, not taking it into account before you design your webpage-- how people will get to your webpage. In detail. (What did they see and how did they happen to see it? What did they click and why did they click it?)
5. Not understanding that trying to please everyone pleases no one.
6. Being afraid to repel visitors. (Even though it was likely designed by some, U.S. Small Business Administration website probably repels people who say "Later, Dude." America Online is actually proud that nerds hate America Online. XXX.com is designed to repel your grandmother.)