As an example, a pure-SEO approach neglects important factors such as stickyness, traffic from social sources and the effects of offline marketing...
Just simple, basic usability could well be added to the list, with reasonable page load times.
What you will find in reality is that any good SEO professional will look at the bigger picture.
Unfortunately, the reality is also that a client has to accept the fact that it goes beyond "tweaking pages" for what they see as their important couple of keywords, and that other factors will affect their bottom line. Some don't (and/or won't).
Also, some won't realize that what they, themselves (or their webmasters) do to their sites, either on-page, site-wide (like internal linking), or regarding their linking profile, can and will impact rankings and then will need continuous "damage control" measures taken.
For someone thinking of entering the field, the other side of the coin could be the question: "What makes a good client?"