Leonard E. Burman (Syracuse University, Maxwell School), Health Mandate: It's Just a Tax Break in Disguise (CNN Money):

I'm doing my taxes and thinking about mandates. That is natural because health reform passed during tax season. Critics argue that the new law's requirement to purchase insurance or pay a fine is a radical departure and unconstitutional.

In fact, this is nothing new. Our tax returns are full of implicit mandates with huge penalties — in the form of lost credits and deductions — for noncompliance. The government wants us to donate to charity, own a home, save for retirement, adopt a child, buy a hybrid car … If we don't, we pay more tax (a penalty).

There is a semantic difference between the health insurance mandate and these other tax nudges in that the government doesn't require you to donate to charity or own a home. But the government doesn't really require you to get health insurance either. You are free to ignore the "mandate" and pay the tax.