Talking about SEO, I’ve found myself with Search Engine Watch. Here I found a very interesting article about SEO, according to their experiences, from the most common mistakes that happen in SEO.
They are:

• A bad information structure
• Poor useability
• Internal link bad management
• Hiding the content of the web page
• Incorrect site addressing
• Bad titles and sub-titles
• Bad content
• Replics from other sites
• Flash all over the place
• The same keywords for all the articles of the web site.