3. Mystical belief in the power of Web Standards, Usability, and tableless CSS. 9. Forgetting the purpose of text.
As a perfect example of #9 we have Driven Technologies. Note the front page composed entirely of graphics. The lack of text makes the site meaningless to search engines. If you heard the company name and did a search you would not find the company. When Google is asked specifically about the domain nothing comes back. The lack of text means that despite having a website, the company is practically invisible on the web.
Of course this mistake should be counted against whomever put together the website, not the company itself.
April 8 update: the above site-specific Google query originally returned just a single empty result. Now there are six entries of which only two have associated text, and all of which were crawled by Google on April 5 between 00:00 and 03:00 (two days after the original posting). Since Google crawls this weblog every day, it is at least possible that the mention in the weblog prompted Google to crawl the site more throughly.
There is something amusing about the fact that a mention as a bad example enhanced their visibility with Google :).