Sustained, non-trivial economic growth.

The twentieth century appears is [sic] unique in its pace of economic growth. Such rapid growth in standards of living has never been seen before, anywhere--save possibly in the generation that saw the discovery of fire.

Between the invention of agriculture and the commercial revolution that marked the end of the middle ages, wealth and technology developed slowly indeed. Medieval historians speak of centuries and half-millennia when they speak of the pace at which key inventions like the watermill, or the heavy plow, or the horse collar diffused across the landscape. [...]

So slow was the pace of change that people, or at least aristocratic intellectuals, could think of their predecessors of a thousand years before or more as effectively their contemporaries. And they were not far wrong. Marcus Tullius Cicero, a Roman aristocrat and politician of the generation before the Emperor Augustus, might have felt more or less at home in the company of Virginia planter Thomas Jefferson. The slaves outside grew different crops. The plows were better in Jefferson's time. Sailing ships were much improved.

(I am less sure about DeLong's remark, which I've excised, that before the Industrial Revolution, living standards were kept firmly in check by the Malthusian trap. The basic conclusion that pre-industrial economic growth was glacial nonetheless stands.)