I've tried to chew a bit on Bryan Caplan's post about why he is not a left libertarian before I raced off to refute it point by point. That's because I suspect no refutation is necessary; Caplan throws into rather stark relief precisely why left libertarianism has more to do with attitude and temperament than blatant differences on principle.
Caplan argues over and over in his article that certain left libertarian arguments do not make sense because, if you consider the issues from an economic point of view, everything balances out. In doing so, he glosses over a key difference between his approach and that of left libertarians generally; many of us find the typical libertarian reduction of all matters of justice, culture, etc. to economic calculation totally warped and inaccurate. More and more of us are rejecting a rigid market fundamentalism that seems to discount any issue that cannot be modeled economically. There's more to human flourishing than marginal value.
For example, Caplan may be right that we effectively forego a fairer rental agreement with our landlord in exchange for lower rents. It's certainly an elegant argument that provides a clean explanation for our entrance into supposedly free contracts with such little negotiating wiggle room for ourselves. Of course, for economists, it's all about explaining within the constraints of the model - there's no price one can place on human dignity, the social effects of systemically lopsided contracts, etc. A pure economic argument does not address whether justice is served, or why people place such a low priority on being treated fairly. It doesn't attempt to back up the speculation on people's motives for accepting skewed contracts with evidence; the mere assertion that our market system has mediated this contract is proof positive that it is fair and balanced, and so the only task left is to come up with explanations for why we chose such an arrangement.