While this is a true and good answer, it still doesn't help you specifically with what not to do. The ?bad? list or not so safe ranges from things that make it hard for a search engine spider to crawl your site to things that can get you banned.

1. Entirely JavaScript / flash site a bad idea.

While some JavaScript and flash add-ins can make for a great web experience, making a site entirely JavaScript or flash will make it hard for a search engine spider to crawl. This will likely result in not getting the search engine results ranking you really deserve

Use regular HTML for links, and a sitemap to ensure the website can be crawled.

2. Avoid excessive graphics and flash presentations.

The reason for this is two fold. This is purely about visitor experience. If a web page takes too long to load (more than a few seconds), the average visitor will quickly hit the back button and you've lost your potential visitor. On this same token avoid very large graphics, instead using thumbnails and allowing the visitor to see a large version if they desire.

3. Avoid the ?quickly get submitted to thousands of search engines? deals.

While many of these do what they say, keep in mind that 98-99% of all search engines are powered by Google, Yahoo, or MSN. Just keep these 3 in mind and you're covered.

4. Don't submit your webpage to search engines too often.

Submitting your webpage urn to search engines if it's new is considered ok (though with backlinks not necessary). Search engines prefer to find your website through backlinks since that is how your webpage gets it reputation and relevance.

5. Avoid keyword stuffing / spamming.

Avoid the temptation to use tons of keywords on your webpage only to try to impress search engines and get a higher ranking. Your search engines results ranking depends upon dozens of factors, only one of which is your webpage content. Write your webpage for visitors with your keywords in the back of your mind and you'll be in good shape.

6. Avoid hidden text.

Hidden text is where a webmaster tries to hide keyword stuffing in plain sight, with a font color the same or nearly the same as the background. This gears it only towards the search engine, and search engines are very adept at catching this. This is the old trick that everyone knows and can very quickly knock your site ranking to near zero.

7. Avoid cloaking.

This is when the webpage your give to visitors is different than the one you give to search engines. Normally the one given to search engines is perfectly optimized for them, but search engines obviously don't like this as the page they index and serve is different than the one that exists for visitors in reality. This can very quickly get your site banned forever from the search engines.

8. Don't forget, backlinks make the site.

This is sometimes overlooked, but a website with backlinks plays a small part in the internet and appears to have less relevance. While how your site is ranked for search results has a lot to do with your content, your link popularity is a major part of how important your website is perceived to the search engines. Miss this one, and your traffic will suffer until you right it.

As you can see, the list of things to avoid can affect your site from appearing invisible on the internet, to getting outright banned by search engines and knocking out 80-90% of your traffic. In general, try not to misrepresent your website to the search engines. Let them index what visitors see, and don't modify your website to try to trick the search engines. Just run a site with good content for visitors, and get quality backlinks, and you're on the right road.

Most people know what things to avoid in the SEO (Search Engine Optimization) game. However, many do not fully know what is over the line and to avoid like the plague.