• assume that a professional-looking website can be done by a non-professional
• regard your website as an expense rather than a business investment
• neglect the textual content
• ignore the latest web design trends (please see my list of the best of web)
• fail to have a marketing plan
• expect that your website will acquire good rankings on the search engines (Google, Yahoo!, etc.) by itself, without an extra effort on your part. Recommended reading: enzinearticles.com.
• ask for the cost of a website without providing the details: number of pages, interactive forms, e-commerce, number of products, etc.