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.
While it sounds like an ideal income stream, there are more specific benefits of residual income. For instance, unlike a salary, someone does not need to remain tied to the same location in order to earn income. He can move halfway around the world and still make the same residual income as he would if he stayed in the same location as his business.
Passive income is the Holy Grail for online marketers. It's automatic. Effortless. But, not at first. In the beginning, it's grueling. I liken this to doing the most amount of work for the least initial return. However, over time, as your passive income begins to increase, your reliance on an active income plummets. That's when the real magic starts to happen.
And speaking of not selling to everyone, don’t try to create products that are trying to speak to everyone. Don’t be afraid to get specific. For example, I’m no longer buying “how to make money blogging” courses that are meant to include beginners. Why? Because I can count on 75% of the content, if not more, to be about things I’ve already done – buying a domain, setting up hosting, setting up this, that and the other thing. I’m spending money on things specifically created for people at my stage of the game. And guess what? That creates a lot of opportunity for you in creating additional courses! You can create one for the beginner, intermediate and advanced user. That's a lot of income potential.
One of the primary benefits of residual income is that it takes little continued effort to maintain. Passive income includes things like royalties received for creating an intellectual property such as a book, advertisement payments received for Internet traffic on websites or content you create, dividends paid on stocks you hold and rent payments. Creating residual income often takes a considerable amount of initial effort, such as writing a book or article, creating a website, buying a building and renting it out or researching and purchasing dividend-paying stocks, but after the initial effort, you receive income over time with little or no additional effort. This can allow you to pursue other opportunities while continuing to earn income based on past efforts.
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.