Quality is key when it comes to content, but so is keeping the amount of data on each page down to a reasonable 5-10kB. This keeps the load times manageable for both broadband and dial-up users.
For keywords, Cutts' posts recommended a strategy of "Once in the title, once in the description tag, once in the heading, once in the URL, once in bold, once in italic and once high on the page." The post also goes on to explain the necessity of a site's internal links.