Residual income is the amount of net income generated in excess of the minimum rate of return. Residual income concepts have been used in a number of contexts, including as a measurement of internal corporate performance whereby a company's management team evaluates the return generated relative to the company's minimum required return. Alternatively, in personal finance, residual income is the level of income that an individual has after the deduction of all personal debts and expenses have been paid.
A very thoughtful list here. Another relevant book published this year is “Retirement Planning for Young Physicians” by Dr. Ralph Crew. It covers many of the topics discussed here from the perspective of a physician who has successfully saved and retired. The book adds a lot to these discussions with a focus on the importance of lifestyle choices, as well as a realistic (though sobering) view of likely future physician income trends and how to plan accordingly for retirement.
Basic marketing ability -- You will not earn a decent income from most blogs if you don't market that blog in some way. That doesn't mean you have to market aggressively or feel like you're whoring yourself out with a constant sales pitch (more on that myth later). The most important thing you'll do marketing-wise is actually completely on the back-end -- evaluating your stats, testing ad placements, and just overall optimizing the site.
Just because the whole notion of passive income isn’t all that passive, doesn’t mean you can’t build monetization into your WordPress site with minimal effort. You’ll still need to participate in all of the general site upkeep tasks like writing new posts, marketing, and site maintenance, but the money-making approach you take can be quite simple and require little work on your part once set up.

The second big takeaway for me, was to pick a niche you know, you find interesting AND where there is demand for your product – ie willingness to buy/pay.  A lot of people only remember the first part of this 2-part formula, and end up creating a blog on knitwear for kittens.  But it’s the overlap of passion and profit that Ferriss stressed so much.