I've been researching Nozick and his commentators for the MA course I'm giving next semester on Contemporary Political Theory (details on my university web page, see right hand column). The most vicious critics of Nozick are certainly his fellow Libertarians, including Murray Rothbard who Nozick refers to as important in converting him to a Libertarian point of view. Libertarian in this context means the capitalist version in which if the state exists at all, it should only exist to uphold property rights based on voluntary contract, and protect individuals from violence. In the Anarchist, or near Anarchist version, of which Rothbard is the best example, these laws emerge in a voluntary way without any need for a state.
Though I was already acquainted with the idea that Capitalist Libertarians/Anarcho-Capitalists are a quarrelsome lot and that most of them are on the fringe of the academic world, I was startled by the response of Libertarians to Nozick. Nozick is by some way the most distinguished representative of that point of view in academic philosophy. No one has replaced him in that role since his death, and Nozick may himself have stopped filling that role. Though he did not say much about political philosophy after his Libertarian masterpiece Anarchy, State and Utopia, there are indications he pulled back from his claims in that book to a softer form of Libertarianism (presumably heading towards the kind of welfarist liberal/capitalist libertarian crossover I favour). The Libertarian response is to sneer that he turned into a social democrat. Anyway they did not like his book in the first place, and were probably relieved that he could be later labelled as an apostate .
Murray Rothbard, and his followers, express great jealousy of Nozick's success, claiming that Rothbard's Ethics of Liberty (click for pdf download) is a more important book. The book can be found via www.mises.org as can a great deal of other relevant material. I certainly don't fault the Misess Insittute people for failure to use the Internet properly. Anyone who compares Rothbard's book with Nozick really ought to feel embarrassed for Rothbard, and his followers, that they could be so self-deceiving and foolish as to think Rothbard's book is better. His method of argument is constant restatement of the view that the state is unnecessary, and that left to themselves, people will create better voluntary arrangements. His method of dealing with different points of view is to insult them and to fall back on an analogy between the state and a supplier of goods or services in a market economy based on rules of a kind which have always been enforced by the state. Just like Anarcho-Communists, Rothbard relies on natural intuitions of Natural Law to substitute for the role of the state. Again the universality of these intuitions is asserted rather than argued for. The fact that Nozick's book is composed from a variety of detailed arguments for his position is used against him by the Rothbardians, apparently people just read those arguments separately which is supposedly easier than reading Rothbard's book through. There is nothing difficult about Rothbard's book apart from the boredom resulting from his constant under argued assertions.
Other criticisms of Nozick from team Rothbard, and other Libertarian crews, include outrage that Nozick finds paradoxes at the foundations of Libertarianism he has to try to answer, essentailly the classic paradox of explaining how people consent to a common set of rules without force. In their zeal Libertarians are shocked by the possibility of paradox in the foundations of their ideal system, though one might think the whole point of political philosophy is to deal with the paradoxes that human reason throws up and which every inquiry into the heart of a subject always throw up.
Jealousy is never far from the surface. The feeling that famous universities are dominated by cliques of elite left-liberal academics excluding the knights of Libertarianism is a constant theme. It is not possible that the left-liberals (also Neo-Conservatives and Conservative Paternalists) could be doing honest work of high quality. The fact that Rothbard never had a job at a famous institution clearly embittered him and his followers. Rothbard appears to have been a generous and inspiring person in some respects, but somewhat lacking in a sense of proportion about his importance and the quality of his essentially polemical work. Nozick was a professor at Harvard, and even worse is very generous about the work of his famous left-liberal colleague John Rawls. Generosity to non-Libertarians is not widespread in Libertarian culture; they find it hard enough to be generous to each other. Nozick appears to have been a sensitive, understanding and well rounded individual who did not try to dominate other people or establish a clique of loyal followers. He was certainly a misfit on Planet Libertarian