The reason for excluding figures such as Locke is precisely because they bridge more than political tradition. That's what we meant by excluding works that are "ambiguous." If a book is seen as seminal by two or more current political traditions, its presence in the curriculum doesn't indicate anything in particular. Likewise, Plato's Republic and numerous other works of great importance to political theory per se do not stand as works that exemplify particular contemporary political viewpoints. We chose 1750 as the cut-off date precisely to avoid these entangling questions of intellectual history. The further back we go before the Enlightenment, the more entangled things get. Locke can be equally claimed by liberals and conservatives, for example, and is therefore irrelevant to this project. I am surprised by the vehemence of some respondants and commentastors who think the whole idea of asking this question, or limiting the date range is preposterous. What gives? Why is the question so threatening? And why is it so difficult to grasp that we would need to first identify important works that are not claimed by more than one current political outlook? As to the list of suggested names, we added it as an afterthought just to start people thinking. Generally it succeeded, but it also seems to have irritated some readers who invested it with sugnificance it didn't have.
1. Coming at the question as a political theorist, the fact that both libertarians and liberals might want (or not want) to claim Locke is something rather telling about each tradition. If, for example, one wonders whether there can be liberaltarians at all--that is, if libertarians must side with conservatives against liberals because conservatives support smaller government--the fact that liberalism and libertarianism share some common intellectual roots is important. I don't know that one needs to go all the way back to Aristotle (in fact, I said in the comments below we don't), but liberalism's history goes back a bit further.
Further, one can, as Alan Jacobs pointed out in his post, use particular figures in more than one way: Locke can be read as proto-American liberal, as the figure Nozick makes him into, as a natural law contract theorist, or as a apologist for capitalism and empire--that is, he can be grist for the liberal, the libertarian, the conservative, or the radical.
2. Following on the second point, my questioning of the list wasn't meant to be a criticism of the list per se, but rather of the idea of assigning particular figures within a tradition. von Mises is a libertarian--is he also a radical? The concrete political suggestions of some of the 'radicals' may be more practical than a minarchist politics. Rousseau, for Popper, is some kind of crypto-fascist; for Walzer, he's a visionary republican and democrat; for the reader of the "Letter to D'Alembert on the Theater" he's a cultural conservative; for the reader of "Emile," he's a liberal educational theorist. He can be claimed by all of these groups--or rejected by them. In some ways, of course, this makes certain figures more interesting: Niebuhr is fascinating because he doesn't clearly belong in any particular political category (one could say the same for, say, Tocqueville). Are Hamilton and Madison in the Federalist liberals or conservatives--or some combination thereof? Does it matter that Burke wouldn't self-identify as a conservative if 21st century conservatives are the ones who would claim him?