It’s never just one thing. 
When your website is successfully driving traffic to your business, it’s not because you did just one thing right. It’s because you did many things right. Your website has to be structured properly and optimized behind the scenes for search engines to find you. Your front end — the content that visitors read — must also be done right. The right keywords is a huge part of the mix, but if you want people coming back to your site again and again, you need content that is compelling and fresh on many fronts.
Stoney Degeyter, over at the Search Engine Guide has a great article on the Best Damn Web Marketing Checklist for Website Content. It lists 23 factors that affect the quality of your content. Many of them have nothing to do with keywords, but everything to do with giving visitors a reason to trust and love your website.