Mollie Hemingway's earlier post about social justice had me thinking about something GK Chesterton wrote in Orthodoxy:
The modern world is not evil; in some ways the modern world is far too good. It is full of wild and wasted virtues. When a religious scheme is shattered (as Christianity was shattered at the Reformation), it is not merely the vices that are let loose. The vices are, indeed, let loose, and they wander and do damage. But the virtues are let loose also; and the virtues wander more wildly, and the virtues do more terrible damage.
As some folks noted in the comment thread to Mollie's post*, there are political acts that are rightly seen as promoting social justice, in the sense of changing social structures so that countless acts of individual injustice are less likely to occur: the end of slavery in North America, the destruction of fascism, and so on.
I think the root of conservative unease with the notion of social justice is anticipated by Chesterton. First, a virtue unloosed attaches itself to things for which it is wholly unsuited. I can understand a prophet railing against the injustice caused by systematic racial bigotry caused by unjust laws (e.g. Jim Crow). It's harder for me to understand an unequal distribution of income as a marker for social injustice, since only in utopia (or in a Communist hell) is it possible for everyone to be materially equal. Social justice demands, rather, that hard work and ingenuity be rewarded materially more than the opposite, which necessarily leads to material inequality.
Second, an inappropriate zeal for social justice causes a lot of harm. I'll leave aside the spiritual harm that comes from the self-righteous feeling to which the socially just are particularly vulnerable -- that problem is best hashed out by a power much greater than me (beyond my pay grade, if you will). The secular problem is the law of unintended consequences. Unemployment insurance, for instance, serves the cause of social justice by providing material care for families who are facing economic difficulties. At the same time, extended unemployment insurance undercuts incentives among the unemployed to look for work. The language of social justice, as typically employed, is unhelpful here since (in order to to emphasize the benefits) it obscures the costs and thus prevents a mature consideration of the issue.
* My apologies to Mollie for hijacking her thought-provoking post, but I couldn't fit these thoughts in a comment box.