So here's the idea: there are at least three different ways that libertarians generally think of coercive force, and I think they've been harmed by treating them as if they were essentially the same sort of thing. The first kind of coercive "force" is the use of someone else's property without their permission, or in direct defiance of their wishes. The second kind is the initiation of physical force upon a person, again without permission or in violation of their explicit desires. The third kind is the threat of physical force directed at coercing a person into doing something against their will. I don't deny that there might be more "kinds," but these three came to mind first, so they're the one's I'll address here.
It seems to me that when many libertarians decry "coercion" or "aggressive force," they do so by means of certain kinds of arguments. These arguments sometimes take the form of identifying the "true nature" of certain kinds of coercive actions, and extending that characterization to other kinds of coercive actions which may or may not fall into different categories of force as expounded above. This, I think, can lead to mistakes.
For example, "Taxation is slavery because the product of your labor doesn't belong to you." The idea here is that the government "steals" some of your income from you (which depends on the rejection of the idea that you voluntarily live on the government's land by living in a country), and this money is the product of your own labor. But you know what you call a system in which you work, but someone else gets the product of that work? Yea, slavery. Blammo, take that government! *high fives from the libertarians*
Except here's the thing. There is exactly one sense in which taxation is like slavery, and it's not the sense that these people are talking about. The way that taxation is like slavery is that the government demands that you, or your employer, or someone not actually on their own staff, actually sends them the money. That is, if you don't put the check in the envelope, stamp it, and send it, you get punished. That's the third kind of force discussed above, where the threat of violence is used to coerce a person into doing something against their will. And that's exactly like slavery.
However, the reason that taxation is like slavery has nothing to do with the fact that the government gets the product of your work. That kind of thinking is not only sloppy, but in many ways anti-libertarian and Marxist. We live in a country where we are told that we will be taxed if we decide to work. In a slave society, slaves are told that they will be taxed and they will work, whether they like it or not. The government does not use the threat of force to make us produce income. That would be like the third kind of force, which is the kind that can be associated legitimately with slavery. We choose to work, and can choose not to do so without any government-enforced consequences.
The idea that the product of our labor should belong to us, even when we are explicitly a part of an arrangement in which it will be taken from us, is an idea that can be traced back to Marx and the labor theory of value. The very same idea is used to explain why capitalists exploit their workers: the workers produce, and the corporation gets the product of that labor. It's like wage-slavery! *high fives from the socialists*
The difference between a voluntary agreement to work and government taxation is not that corporations let you keep the product of your labor and the government doesn't. Rather, the difference is that you don't agree to the arrangement the government enforces upon you (well, under certain conceptions of voluntarity), and you do agree to the arrangement the corporation enforces upon you. In a corporate agreement, you are "paying" your employers with the product of your labor in exchange for them paying you with money that you want more than the product of your labor. In a government agreement, you are paying the government with the product of your labor so that they won't use the first kind of force (the use of your property against your will) or the second kind (the use of physical force against your person).
The slavery part of that bargain is the part where they actually make you pay the tax with the threat of those other kinds of force. If they just went to your bank and took the money from you, the slavery component would be gone. But ostensibly, nobody's complaining because we have to put a check in an envelope and mail it, or fill out a form every once in a while. I mean, that's a pain in the ass, but I really don't think that's the problem. The problem is not that taxation is like slavery, but rather that taxation is like theft.
But if you accept the view that we voluntarily live on the government's land, and part of our contract is that we pay taxes to the government, then even this characterization doesn't apply. The first kind of force (using property without the owner's permission) doesn't apply to situations where the alleged "owner" is party to an agreement through which she surrendered the title to that property, just like it's not the first kind of force when my employer takes the product of my labor without my consent. And the second kind of force (physical force against someone's person) doesn't seem to apply when the person is doing something that they have a perfect right to be doing, and are resisted against forcefully. In a sense, it would almost be self-defense for the government to collect its "rent", just like my employer would have every right to use some reasonable amount of force if I suddenly refused to give up the products of my labor in the middle of the work day (well, more accurately, he would have the right to demand that I appear in court or something similar, and if I were found to be in breach of my contract, some force would be legitimate in enforcing that contract; I'm no opponent of procedural justice).
So we see that "Taxation is slavery" collapses into "Taxation is theft, and they even make me give it to them at gunpoint", which then collapses into "The government doesn't have any legitimate claim to my property." It seems to me that all that would be perfectly clear to anyone willing to think about it for a moment if they would only separate the different kinds of force and consider each in its own light.