[gabemart]

The most difficult part of SEO (and, perhaps tellingly, the part covered only extremely briefly and superficially in the PDF) is getting links from relevant, high-authority sources with relevant anchor text. It's more important than any other factor by a vast margin. Building image sitemaps or optimizing your custom 404 error pages pales in significance by many orders of magnitude. On-site SEO is child's play compared to offsite SEO.

Getting these links ethically is a hard problem to solve, even for a high quality site with unique content.

I don't have much connection with the world of commercial SEO, but I imagine there are people good enough at solving this problem to charge a premium and get good returns for their clients.