One is the frictional cost of running a system like this--paying the salaries of the tax lawyers, the lobbyists, the hardworking bureaucrats like Sciscioli.
The prof's New Economy spin was that frictional diminishing returns did not apply to software or Internet firms because of their tiny marginal unit costs.
There will probably always be some frictional unemployment even in good times, but individual incentives on the supply side will keep pressing for employment individually and toward full employment collectively.