In its simplest form, on-site
SEO pretty much are multiple sets of rules for accessibility of users and spiders. The off-site
SEO seems to be more customized. Utilizing social networking, link baiting and any number of other ways to gain effect back links is the creative part.
Now some websites simply already have plenty of links. If that is the case then it comes back to the on-site factors. You start looking at how users interact with the website, what is the shopping cart experience like? What pages are users spending the most time on. Why? Etc, etc, etc.
When I first took at a prospects website all these various things are flying around my head.