"Social justice is the application of the concept of justice on a social scale."
Does it mean that groups (on the "social scale") are granted the same rights as individuals? I.e., make corporate [1] personhood a fundamental concept rather than a convenient interface?
The only really concrete idea I get from the wikipedia article is that an unjust government is illegitimate (without explaining what "unjust" means), and that somehow equality of outcome is desirable.
[1] By corporate, I mean any large explicitly defined group of people, not strictly limited liability corporations.
I am sure others would include additional programs in the definition, but I think that anything crossing the line from providing equal opportunity to equal outcomes (ie quota programs) crosses the line of equitable treatment in the other direction.
Also, I'm not sure how OSHA-type protections support equality of opportunity. How does removing the ability to negotiate workplace safety with one's employer create equality of opportunity? And also (more to the point), how does progressive taxation create equality of opportunity?
I think social justice is more about equality of individual justice as applied to people in different classes. I guess that's how I'd answer that.
So OSHA requires protections for low-"class" workers that rich people would do if they were doing the work... something along those lines.
He acknowledges your point that the debate on flat vs progressive tax rates is a virtue vs social justice debate.
He also is debunking the idea that higher taxes hurt the economy, which is the OMG BE SCARED argument that the right uses to try to lower taxes (without a commensurate reduction in spending) in order to grow the economy and (eventually) increase the tax base. I recently wrote a similar article about this particular point (http://www.reasonablyopinionated.com/2010/02/tax-revenues-ta...) because the only thing that ever happens in modern history when taxes are lowered is horrible deficits. As the Slate article points out, there isn't a strong correlation between tax rates and GDP growth.
Personally I am on the side of Buffet and Spitzer on this issue and do support progressive taxation, precisely because of the social justice argument. I believe in equal opportunity, but abhor the idea of forcing equal outcomes.
What really irks me about the anti-progressive-tax crowd is how they don't even acknowledge the benefits the rich get from such a system: efficient and reliable infrastructure, political stability, and a large middle class (ie customer base). If the government did not make those investments, the overall pie would likely be smaller.