The self-employment tax is particularly nasty because it's regressive: People who have high-paid (ie. over ~$80K) jobs but do some extra work on the side don't have to pay it, whereas the genuinely self-employed do. Combined with state taxes, it can easily drive the marginal tax rate to >50%.

The other big cost is health insurance. Not technically a tax, I know, but the effect is the same.

I believe everyone is taxed on the first ~80K.  Those high-paid people do pay the tax (their employer actually pays it).  But if their employer paid them as a contractor, the employer could afford to pay them more (their salary+the payroll taxes).  Uncle Sam gets it one way or another.
Health insurance isn't a free good; it's part of your total compensation package. It's always better to look at your total compensation (salary + employer's social security contribution + benefits) rather than your salary alone to determine how much you get paid.