Tyler Cowen and others are talking about the spectre of zero marginal product workers. Now I'm not precisely sure how he is using the term but I take it to mean that they can't produce enough of value to make it worth hiring them - given present circumstances. That seems almost tautological to me. If somebody could make money hiring them, why wouldn't they?
The unemployed tend to unskilled or old, and they tend to live where a lot of other people are unemployed.
It's not to hard to imagine a world, or at least part of a world, where most people don't have anything of value to sell. If most goods are produced by automated machinery, or by those willing to work dirt cheap on the other side of the world, many kinds of labor lose any value. For the time being, the wealthy and their better off minions may want or need various forms of service from the comparatively unskilled masses. Judges and policemen to protect their wealth, a few servants for style, and soldiers to protect them.
In a free market, those with nothing valuable to sell could starve in world producing thousands of times as much per capita as it did 1000 years ago.