1) Overuse of Flash. Google recently announced that they can now index flash websites but it's still not advisable to design whole websites or the main navigation in flash.
2) Ignoring Title Tags. Title Tags are possibly the most important element used by search engines to determine the topic of a certain web page. Ignoring title tags or simply inserting your company name as the title will ensure that you won't rank highly for any competitive keywords. Similarly, ensure that each web page of your site has a unique title tag.
3) Using Images as Headings. Keywords are still very important when wishing to rank highly for keywords and inserting your headings as images will result in search engines being unable to read the title and therefore determine what it says. This is an important opportunity lost as titles frequently contain your target keywords.
4) Ignoring URL structure. Including keywords within your URL structure will help to give your web page a boost in search engines. Also, don't use dynamic URLs which contain many elements such as equal signs or question marks. Static URLs are much easier for Google to crawl.
5) Internal Linking. This is one of the most frequent mistakes, yet internal linking is one of the best ways to improve the rankings of the different pages within your site. Make sure that each web page isn't further than two or three clicks from your home page. Also, always link to pages with your target keywords rather than 'click here'.
6) Poor Keyword Placement. An added emphasis is placed by search engines on the first 100 words of a web page. Ensure that you place your target keywords and keyword phrases within this content.
7) Same Meta Description and Keywords. Although search engines tend to ignore the meta description and keywords when it comes to ranking sites, they do still consider them when determining what pages contain duplicate content. Using the same meta description and keywords across multiple pages increases the likelihood that some of your pages will be filtered out by search engines.
Duplicate pages. Make sure that search engines only index one version of your website, otherwise you can end up receiving a duplicate content penalty. For example, don't have your website appearing on two different URLs - redirect one to the other.
9) Using incorrect redirects. Sometimes a web page moves location and its previous URL must be redirected to its new location. Always use a 301 redirect to do this. 302 redirects are used for temporary redirection and can result in a Google penalty.
10) Buried Content. Over time many websites and blogs create a huge amount of content which simply ends up being buried within the site. Use internal linking and a clear navigational structure to keep all the content within your site a maximum of three clicks away from your homepage.