Accretion/dilution analysis Adjusted present value Associate company Business valuation Conglomerate discount Cost of capital Weighted average Discounted cash flow Economic value added Enterprise value Fairness opinion Financial modeling Free cash flow Free cash flow to equity Market value added Minority interest Modigliani–Miller theorem Net present value Pure play Real options Residual income Stock valuation Sum-of-the-parts analysis Tax shield Terminal value Valuation using multiples
That income is considered residual income because as long as the apartment is rented and the rent is collected, the income is earned without additional effort. The effort came when the property was purchased and a tenant was found. Each month after that, the money automatically is paid without buying the apartment again or finding the same tenant each month.

One of the things I'm surprised your article doesn't mention is the tax advantages of this type of investment. The depreciation and rehab costs (purchasing distressed properties) can be huge deductions to ones income taxes, which none of the others have. Then, along with the appreciation of real estate, this passive income investment outperforms the notion of maxing out my 401k as well.
I am an English major and a herbalist with so many ideas and no extra income to fulfill them. I recently started renting my extra apartment in the attic with Airbnb. It’s amazing how fast I accumulated some money for few hours of work between guests. Now I want to persue all my dreams of opening an online herbal store, publishing my ebook of treating Ulcerative Colitis with herbs, blogs, and videos, and pretty much all of the ideas mentioned here. I will save this article as its really helpful for whomever needs some ideas…

I prefer assets that make me a high return for the lowest amount of work possible (semi-passive involvement). And assets that pay me in several unique ways. Cash flow is only one way RE makes money for me. I also get principal reductions, appreciation, tax advantages (depreciation), and I control the rental increases on a yearly basis. Plus a majority of the capital is provided by the secondary market on 30 year fixed low interest rate debt.
First: I understand why you would say that such investments are restricted to only accredited investors, because generally, that’s true. There are means, under federal securities regulations and Blue Sky laws in each state, to sell interests to non-accredited investors – but usually those means are so heavily regulated and involve disclosures so similar to cumbersome registration requirements that it is not worth it for the seller to offer to non-accredited investors.
If you're looking to get started in real estate, look at a crowd funding solution like RealtyMogul. It works similar to LendingClub - you commit as little as $5,000 towards a property. When the property is fully funded, you become an owner, and will receive your share of the earnings and appreciation in the property.  Check out RealtyMogul to learn more.