Little or no useful content. Visitors won't stick around unless there is real content and not just a simple sales pitch.
Misspelled words, typos, and bad grammar. There is nothing more unprofessional than having speling and other language erors on a website.
The "too many" syndrome. Too many colors. Too many font sizes. Too many "cute" graphics that don't add value and increase download time.
Gratuitous Flash animations. Some animations are put on websites for no reason than to impress other web designers. An uncluttered, well-organized website will have a greater impact than one that uses intrusive animations that distract from your message.
Huge company logos, meaning the site is all about them, not about the customer.
Bad color combinations that make you dizzy or background graphics that make it difficult to read text on a page.
No apparent means to navigate through the website or to return to a previous page. Easy-to-use websites have a logical structure and use navigational elements in a consistent manner.
Not optimized for search engines. Many websites are not using titles and keywords correctly, or contain code that penalize their rankings in the search engines. Search engine optimization is extremely important if you want anyone to find your website.