I liked this article, except for this part: "...his home country, where the middle class is burgeoning but almost half the population still lives in poverty." This is a naive claim, it may appear so because people here in Mexico (specially in urban areas) spend a lot on bars, restaurants, vacations, gadgets and even cars. What the author oversees is that this same people have not enough savings in the bank to survive…
Some people define it via income characteristics. So a college graduate, with no house or car, can be middle class if they land a good job.
Some people define it via wealth, so a retiree with no income can be middle class if they own their own home, and have a decent amount of savings.
Some people use the 'classical' (ala Marx) definition, in which the middle class are the socio-economic group that can buy the labour of working class, but typically work along-side them. For example, a mom-and-pop shop would have middle class owners, and working class employees.