As George H. Smith points out in the post over at Libertarianism.org, libertarianism properly understood embraces an ideology of negative liberty as opposed to the the idea of positive liberty: Negative and Positive Liberty. Positive liberty is the idea that freedom is primarily about choosing to do good and right, rather than just doing whatever it is you would like to do at any given moment. Smith's argument is that libertarianism is primarily about negative, rather than positive liberty.
This to me hits the problem with libertarianism right on the head, although Smith doesn't view libertarianism's embrace of the negativism as a problem. Negative liberty is the idea of freedom from constraints on personal impulses and desires. Balancing such an individualist approach to freedom, positive liberty is primarily concerned about freedom being oriented towards the classical virtues of truth, justice, temperance and fortitude. To put it more concisely, positive liberty is freedom for something, while negative liberty is about freedom from something. In classical liberal and conservative theory, positive liberty correlates strongly with ideas of civic virtue and duty. Negative liberty, on the other hand, simply degenerates into license.
For civic order to be preserved and for human beings to properly flourish, our impulses and desires must be tamed by virtue and duty. Thus, both forms of liberty are necessary for a truly free society. Classical liberalism understood this, and modern conservatism understands this as well. Sadly, the mainstream of modern libertarianism -- closely allied with many on the cultural Left -- does not.