One of the features libertarianism (propertarianism) shares with neoclassical economics is that it tends to take the existing economic system as a given, and proceeds to analyse from there. The result is that much of what follows could be labelled as question begging: incidence of market failure do not merely beg the question ‘how can we fix this?’ but also ‘why are there so many of these?’ Questions over ‘human nature’ become questions of ‘how humans behave under capitalism.’
Neoclassicism’s failure to address any questions about capitalism as a whole is a major flaw, and libertarianism – sharing, as it does, many intellectual similarities with neoclassicism – carries over this flaw. The result is that libertarian analysis, even when cogent, fails to ask truly difficult questions.Unlearning Economics — Musings on the Current State of Economics
Are Libertarians Just Lazy Marxists?
We've hashed and rehashed many of these arguments here in the comments, but UL summarizes it nicely.