It goes without saying that communication skills are a underlying necessity in all forms of RE investing.   

It also creates a necessity to maximize rents to maintain a semblance of value and cover the increased tax burden, especially for properties that have run out of depreciation.The value of properties actually drop, because they lose utility in terms of exit strategy, so it degrades current wealth.It also eliminates a path to wealth for anyone who isn’t already wealthy, thereby decreasing upward social mobility.If there is no step up in basis and an elimination of the $11m margin before inheritance tax kicks in, properties will be forced to be sold to pay inheritance taxes, so there is no way to create a legacy.  

Honestly a mortgage is the only thing I would put in the first category and even then, I'd say it is a necessity because in the long term owning your own house is a good thing. 

Patience and reserves are necessities.

Otherwise you will end up like many people who buy at the top of their budget and become house poor.Getting a value add property is also a necessity. 

Downsize.Cut out everything besides absolute necessities and get your spending down below your income and keep it there.If you or your family refuse to be satisfied with the basics while you get things in order.

“Necessity is the mother of invention.”― PlatoFor those of you thinking " ______ is good enough for my market then."

I think Sub2 was born out the the necessity to sell highly leveraged property.

One bad wholesaler doesn't diminish the necessity and usefulness of our service.

My story: husband and I started flipping and selling manufactured homes in 2013, out of necessity.