Career Capital is the essential currency which can lead directly to fulfillment and happiness if “spent” appropriately. For example, after several years of doing great work, it would not be difficult to ask for flexibility in schedule, hours, or what specific projects you work on.
Financial Viability is important when determining a pursuit; if you have 0 experience in your “passion” you should not expect to be financially stable in any reasonable amount of time. Example provided was a marketing executive who quit her job to start a yoga studio with virtually no experience. As you would expect, it did not go well.
The Law of Remarkability; if you want to earn career capital, you must do or work on things that are literally remarkable. That is to say things others would remark about/upon.
Control is the universal metric that most often leads to satisfaction and a feeling of contentment. Following your passion on the other hand, is a fools errand. Notable example: the farmer who spent over a decade building career capital “on the side” and eventually built and managed his own successful farm. This was not originally a passion, but grew to one as the feeling of controlling his own destiny was paramount. Similar with freelance software developer who opted for freelance contract over “climbing the ladder” so as to have control over her schedule and what projects she works on.
The essential summary of Cal Newport’s So Good They Can’t Ignore You is actually quite obvious: follow your passion is in fact terrible advice. On the other hand, if you are willing to do good work that is remarkable, and spend the time investing in yourself and your skill(s) you will earn career capital in literally any pursuit such that you could be quite happy and fulfilled with it. The feeling of being good at something and it having meaning and purpose far outweighs a pre-contrived passion for something you otherwise have no skill(s) in.