What happens if you don’t buy insurance and you don’t pay the penalty? Well, not much. The law specifically says that no criminal action or liens can be imposed on people who don’t pay the fine. If this actually leads to a world in which large numbers of people don’t buy insurance and tell the IRS to stuff it, you could see that change. But for now, the penalties are low and the enforcement is non-existent.
I'm not sure why Congress crafted a mandate with no teeth. Why they didn't just go the Republican route and grant a tax credit to those who buy insurance (equivalent in all ways except legal language to a mandate) is beyond me.