A minor correction: it's not as bad as 15.3%. The self-employment tax is really just the company share of FICA withholding (6.2%) plus Medicare withholding (1.45%) If you're an employee you pay 6.2%+1.45%=7.65% and your employer pays the same. If someone else (employer) is not paying the 7.65% then you do.
However, for 2011 the FICA rate for employees is 4.2% instead of 6.2%, so the self-employment tax remains effectively 7.65%. But that's only for the first $106,800 in earnings - everything above that is not subject to FICA and Medicare for employer and employee.
Well, it's 15% for FICA (employee + employer), but there's also normal income tax, which starts at 10% and gets up to 35%. And then the state and, depending on where you live, the city have income and/or payroll tax as well (usually in the single digits).
And on top of that the state income taxes in most states, and sales taxes that usually approach 8 or 9%, and property taxes on real estate (and autos where I live). Also if you manage to save anything after all that, you can look forward to paying dividend or capital gains taxes (a form of double taxation).
Most other countries have twice that sales tax, as well as having all those other taxes. The best rough estimate of taxation burden might be proportion of government spending of GDP, which IIRC is about 35% for US, and is lower than eg most European countries.
You need to take into account what you get in addition to what you pay.
In American those taxes provide a strong military. Most of the rest of the world are very poorly equipped by comparison. Other countries get health care and education.
Improving foreign policy would be a cheap way to reduce military costs - relying on a militia was the American tradition up until the Cold War and is indeed one of the reasons you have the right to arms.
However, for 2011 the FICA rate for employees is 4.2% instead of 6.2%, so the self-employment tax remains effectively 7.65%. But that's only for the first $106,800 in earnings - everything above that is not subject to FICA and Medicare for employer and employee.