1. Driving on public roads, unlike living, is a privilege to which the government may attach conditions.
2. Americans can avoid the car insurance requirement, but they can't avoid the health insurance requirement. As Northwestern University law professor Eugene Kontorovich puts it, "If you don't want to pay [for] car insurance, you can sell your car, but if you do not want to pay [for 'minimum essential coverage'], you have to kill yourself." The lack of choice in the latter situation—which means the regulated individuals literally have done nothing that qualifies as interstate commerce even under the broadest conception of it endorsed by the Supreme Court—was crucial to Hudson's decision.
Here are two other important differences between state car insurance requirements and the federal health insurance requirement:
3. State car insurance mandates require you to buy liability insurance in case you injure other people or damage their property, while the federal health insurance mandate requires you to insure yourself against the costs associated with your own injury or disease.
4. Car insurance mandates are imposed by states, whose powers, unlike the federal government's, are not limited to those specifically enumerated in the U.S. Constitution. Although states' powers are restricted by their own constitutions and by some provisions of the U.S. Constitution (most of the guarantees in the Bill of Rights, for example), it makes no sense to talk about the limits of a state's authority to regulate interstate commerce.
That last point is pretty important in assessing a decision like Hudson's, which applies the U.S. Constitution's Commerce Clause to the federal government. But it is routinely overlooked by ObamaCare's defenders (including the president), whose car insurance analogy assumes that the states and the federal government are bound by the same rules.