Something significant that this entire thing highlights is how badly lower income people have been screwed over in the US by payroll tax.
Most people understand payroll tax to be a flat tax rate on all your income, and thus reasonably fair. In fact, it only applies to the first $115,000 or so of income. So, those with very high incomes like Romney pay almost no payroll tax, while for all normal people, payroll tax eats a significant portion of their income.
Now, an important thing to remember is that payroll tax is a big deal. It is what pays pensions for the elderly and disabled. Federal income tax will produce revenue of around $1.4 trillion this year, and payroll tax $0.8 trillion. And the very rich basically don't pay payroll tax.
This has allowed Romney et al to complain about people not paying income tax, while failing to recognize the payroll tax they pay -- and Romney doesn't pay.
Remember, payroll tax works out to 15.3% of your income (if you're earning less than $115k or so), and unlike income tax, payroll tax has very little scope for deductions or otherwise avoiding it. How much federal income tax does Romney pay? Well, he won't reveal his tax returns, but he says "at least 13%".