The title of this post is apt for a book, not a blog entry.
Nevertheless, I might as well start getting some of this stuff off of my chest, to clarify the sense in which I am and am not a libertarian.
I count as a libertarian, broadly speaking, because I am a fan of the free market, and of traditional, meat and potatoes, Enlightenment-era civil liberties such as the right to free speech, the freedom of religion, and all that sort of thing. It is also fair to call me a classical liberal or neoliberal.
What's wrong, then, with libertarianism? Libertarianism is primarily a political theory about the nature of liberty and the proper role of government, but many libertarians also as a matter of fact have ethical beliefs that I find suspect. Robin Hanson, though he may be a sex-obsessed misogynist (hey! I said may be), is right about this: an axiom about the value of liberty (as conceived of by most libertarians) should not be taken as the basis for libertarian political theory. The reasons for this are many, but one of them is that the theory risks becoming question-begging. Why is it wrong for the government to restrict a person's liberty? The libertarian axiomatist's reply: because it's always wrong to restrict liberty! It would seem that the putative libertarian axiom is in fact the conclusion which is need of prior justification. Moreover, there is liberty and then there is liberty. Even if the normative value of liberty were quite fundamental to our political theory, we need an argument for why we should adopt the libertarians' conception of liberty, and some alternative conception (such as the Marxist).