2.Lack of consistency and maintenance
If you want to be successful, you need to permanently optimize your site, keep an eye on the competition and – changes in the ranking algorithms of search engines.
3.Concentrating too much on meta tags
A lot of people seem to think SEO is about getting your meta keywords and description correct! In fact, meta tags are becoming (if not already) a thing of the past. You can create your meta keywords and descriptions but don’t except to rank well only because of this.
4.Using only Images for Headings
Many people think that an image looks better than text for headings and menus. Yes, an image can make your site look more distinctive but in terms of SEO images for headings and menus are a big mistake because h2, h2, etc. tags and menu links are important SEO items. If you are afraid that your h1 h2, etc. tags look horrible, try modifying them in a stylesheet or consider this approach: http://www.stopdesign.com/articles/replace_text.
Many people underestimate how important a good URL is. Dynamic page names are still very frequent and no keywords in the URL is more a rule than an exception. Yes, it is possible to rank high even without keywords in the URL but all being equal, if you have keywords in the URL (the domain itself, or file names, which are part of the URL), this gives you additional advantage over your competitors. Keywords in URLs are more important for MSN and Yahoo! but even with Google their relative weight is high, so there is no excuse for having keywordless URLs.
It is a common delusion that it more backlinks are ALWAYS better and because of this web masters resort to link farms, forum/newgroup spam etc., which ultimately could lead to getting their site banned. In fact, what you need are quality backlinks. Here are some more information on The Importance of Backlinks
7.Lack of keywords in the content
Once you focus on your keywords, modify your content and put the keywords wherever it makes sense. It is even better to make them bold or highlight them.