My objections to Libertarianism does not rest on the claim that Libertarian ideology is incoherent or internally inconsistent. Their ideology is as coherent and internally consistent as it needs to be. Unjustified coercion consists of the use of force without a specific, individual agreement permitting that use. Ownership of property is absolute, and its acquisition and transfer is (with a few easily patched exceptions) well defined. Fundamentally, Libertarians (as well as most left-anarchists) believe that social, collective decision-making must be restricted to privileging Libertarianism (or left-anarchism) as a system of individual rights, privileges, and obligations. No other collective decision-making is valid or legitimate. Libertarians want a specific socio-political system, and there's nothing wrong with wanting a socio-political system, and there's nothing wrong with advocating a system because that's the system the advocate wants.
I object to Libertarianism on two bases. First, it's not the sort of system I want, because I believe that because I don't own much property, I would a virtual slave. Libertarianism seems like a very thinly disguised oligarchical plutocracy. I don't think it's a system that most people want.
Some Libertarians are pretty much upfront that most people would not want Libertarianism (or anarcho-capitalism). For example, as Andrew Dittmer interprets Hans-Hermann Hoppe, Libertarianism considers the preferences of the "dull and indolent" masses to be irrelevant and their satisfaction would lead to degeneracy*. However, what torques me up about Libertarians is that there's a strain of pervasive intellectual dishonesty among Libertarians, at about the same level as the intellectual dishonesty of Christian apologists. To a certain extent, neither can avoid such dishonesty: it is difficult to convince people to legitimize social, political, cultural, and economic systems such as Christianity or Libertarianism that are not in their material interest, and all social systems require popular legitimacy.
Hoppe, Hans-Herman. Democracy: The God That Failed, 2001. p. 288. qtd. in Dittmer
The intellectual dishonesty is not in the underlying ideology; the dishonesty resides in the justification for the ideology. Every Libertarian I've ever met starts off trying to leverage our dislike of other people forcing us to do things. So the argument always begins with the use of force. Jeff Orok, State Chair of the Libertarian Party of Colorado used this sort of argument when he was speaking at the Humanists of Colorado meeting: paraphrasing from memory, he said that if it is wrong for someone to take your watch at gunpoint, then it is wrong for the State to take your money at gunpoint. Because this argument is not otherwise qualified, the implication is that the "taking stuff at gunpoint" is what is wrong. Similarly, when I debated Jon Adams at the University of Colorado, Denver, he said (again paraphrasing from memory) that taxation is wrong because taxation entails that men with guns will shoot you if you don't pay. Again, without additional qualification, the clear implication is that what is wrong is that men with guns must collect it. But of course any sort of absentee property ownership requires that the owner be able to take stuff at gunpoint. The argument then changes to whether or not the force is coercive, i.e. whether or not the person subject to force has agreed to be subject to force. This move is problematic in two ways.
First, the notion of legitimizing force by agreement confuses the subjective with the objective. Because Libertarianism is not popularly appealing, one Libertarian claim is that Libertarianism is objectively true: regardless of what anyone or everyone believes or prefers, the set of rights, privileges, and obligations that Libertarianism describes are true, in the same sense that regardless of what anyone or everyone believes or prefers, when you slap a few kilograms of plutonium together, you get a very large explosion. But agreements are inherently subjective; we cannot even talk about agreements except by discovering what people actually believe and prefer. Agreements cannot be "objective," and it is difficult to see how an inherently subjective phenomenon can be regulated objectively. Once we talk about agreements legitimizing force, we have decisively moved to the realm of the subjective.
The second problem emerges from the first: what constitutes a legitimate agreement? Why should we consider one system of legitimizing agreements, Libertarianism, superior to other systems, such as liberal democratic republican capitalism, democratic socialism, democratic communism, state communism, or fascism? Why should we demand individuated agreements exclusively and consider collective agreements, such as taxation, to be categorically illegitimate? The appeal to natural law has to fail. In the first sense of natural law as objective, how can there be a mind-independent law that regulates mind-dependent things? In the second sense of natural law as what the vast majority of people want, independently of their power or positions, the vast majority of people do in fact legitimize collective agreements, so Libertarianism manifestly contradicts natural law in this sense. Indeed, collective agreements are manifestly more efficient where externalities, information asymmetry, finite buyers and sellers, barriers to entry, always-declining marginal costs, sunk costs*, and other factors render markets inefficient.
*Sunk cost is especially important, because capital is always a sunk cost. In an industrial economy, the cost of capital is a substantial fraction of the total cost of most goods and services. Pure free markets push the price of a good to the marginal cost of the last item produced; the cost of capital is entirely contained in the marginal cost of the first item produced. Without some collective agreement protecting capital costs, an industrial economy will collapse.
If the natural law argument fails, then Libertarians have to make a pragmatic argument. When they do so, they fall to the level of pure lies and bullshit shown by Christian apologists. Market-based healthcare is more efficient than socialized healthcare! Well, no. purely privatized healthcare very obviously less efficient that socialized healthcare for most ordinary, commonsense definitions of efficiency. Voucherized education is more efficient than socialized education! The facts are now coming in, and no, it's again obviously not efficient. Latvia's incredible growth shows the economic efficacy of Libertarian policies adopted after the 2008 global financial collapse! But Latvia hasn't grown incredibly; it's still worse off both in GDP and employment than it was before the collapse. (And even if it had, Latvia? Good grief.) The bullshit is just as thick: every partial success that happens is attributed to Libertarianism; every partial failure is the result of the Libertarianism not being sufficiently complete. (Every "miraculous" survival is the hand of God; every tragic death happens despite God's concern.) When they're not just lying about the facts, they're creating patently unfalsifiable standards.
As I said in the beginning, Libertarian ideology is consistent, at least as consistent as it needs to be. Internal consistency is not a high bar: if the Christians can manage it, and they can, the Libertarians should not have much a challenge. The problem is the justification: the justification is equivocal, inconsistent, disingenuous, unfounded, lying, and riddled with bullshit. I can see no other explanation but that Libertarians want an oligarchic plutocracy, and for obvious reasons, they cannot be forthright, so they must cloak their ideology of slavery in the language of liberty. It is unfortunate that so many who would be slaves in a Libertarian society advocate Libertarianism so fanatically, but anyone who has studied Christian apologetics will understand that the tendency of slaves to fanatically advocate the part of their masters is not a Libertarian but a thoroughly human characteristic.