I bet most people will not believe me if I said that there is just one single most important factor when it comes to SEO because the forums are filled with advice, there are $1,000 courses teaching SEO and companies charge $5,000 or more per month for SEO services.
But after 10 years as an Internet marketer, I can confidently say that the most important factor is: LINKS!
In basic terms, the more links you get to your website, the higher you will rank. If you get enough links you can even forget about doing onpage optimization because you will still outrank the competition. The classic example of this is Adobe's Acrobat Reader page.
Head over to Google and search for the phrase 'click here'. Adobe's page comes up number one, beating sites which have the exact phrase in the title, description – it even beats the company who call themselves Click Here who I would assume optimize for that phrase. In fact, if you go on to Adobe's page the word 'click' and 'here' don't even feature, not in the text, not in the source code, so why is this?
It's one of the proofs that knowledgeable SEO experts point to in order to show that back links are the most important factor in getting top rankings in Google. In this case, there must be hundreds of thousands of high authority sites like Government departments and Educational institutions, not to mention some of the largest corporations in the world, who link to the Acrobat Reader software with the words “Click Here” as the anchor text.
The anchor text is vitally important because Google and other search engines look for what keywords are included to discover the relevancy of the site. However, it's also been shown that if you use the same keyword for every backlink you get, then search engines may ban your site for spamming. It's best to use your main keyword about 50% of the time, secondary keywords 30% of the time and then using the site name, full URL link and stock phrases like 'click here', 'more info', 'visit site' to give some variation.
Another important consideration is where you get these backlinks from. Although Bing and Yahoo (which is powered by Bing) seem to consider the quantity of links a site has for the ranking whereas Google appears to be a bit choosy about which links count.
In this case, getting thousands of backlinks from directories might help you in Bing, but will have little effect in Google. Infact, getting just a few links from high authority sites or relevant sites which have a high profile in Google can help just as much as getting those thousands of backlinks, so you may want to focus your energies on getting high quality backlinks rather than relying on submissions to get good rankings.
In closing, although you should do some basic optimization on your website, don't get obsessive over it as it's worth your time building links rather than considering the perfect keyword density!