There are some design elements that should be avoided:
Flash – Search engine spiders don’t like flash files. There are no textual elements in flash i.e. the spiders can’t scan, read them or understand what the pages or the site are all about half as well as they can with html pages. Many new designers make the mistake of relying on flash files because they look great but in reality, all of that content, including your keywords, will not be of much use if they only appear in flash format. Google and other search engines are working on understanding flash better but the technology is not quite there yet. Also, try not to rely on images as navigation - text is much better from an SEO point of view.
Frames – Frames are another design element that is largely ignored by spiders. Like flash, if you feel you must use frames, keep them to a minimum and find another way to represent important information in text form.
Images with Text – Search engine spiders will not index text that is included in images, so it is important that your images do not contain vital text. It is a better idea to use “alt” tags which serve as descriptive text that tells the spiders what the image is or represents; or to use CSS to display text behind an image.
Other non-design related common website optimization mistakes that people tend to make when beginning to optimize their sites for search engines could be:
“Black Hat” web site optimization
This includes techniques that are unethical and not looked upon favorably by search engine spiders - keyword stuffing, doorway pages, invisible text, and many more. These practices are technically likely to raise your site ranking. However, search engine spiders have become very smart to Black Hat SEO techniques and can now detect them with ease. As a result, your site will be poorly ranked and you lose any good SEO that you have done. Don't go for the quick fix; learn how to optimize your site for the long haul.
Using link farms and FFA’s
There is no doubt that link building is a vital part of SEO. But inbound links are only valuable when they are legitimate. Link farms and FFA’s (free for all’s) are used by many sites looking for inbound links, but these links carry no value. Search engines know when links are coming from these kinds of sites, and not only will these sites not provide your site with any traffic, certain search engines will ban those who participate.