Sherry Glied has a nice summary of the issues involved in the Democratic primary debate in the NEJM.  Which is better:  individual mandates or coaxing people to buy insurance with subsidies?  The problem with subsides: 

• To get the job done, the subsidies might well exceed the cost of the coverage itself.
• Subsidies will inevitably crowd out private spending, shifting to the taxpayers burdens people would otherwise shoulder on their own.

But mandates are no panacea either because of three risks: 

• First risk: a mandate is a tax and if government subsidies are insufficient, it will become a very regressive tax.
• A second risk: special interests will bloat the required benefit package.
• Third risk: to be effective there must be continuous coverage and enforcing such a mandate may require a degree of intrusiveness and bureaucracy that many will find unpalatable.

Gene Steuerle once again makes more sense than most other people, either on the right or the left, trotting out a proposal he and I made years ago:

Clinton may be suggesting more of a commandment than she can enforce, Obama less of an inducement than he can provide … Let me offer my solution (again!) for one enforceable way to extend "mandates" … [T]he federal government could make the $1,000 per child credit, which most taxpayers receive, available only for those who buy insurance for their children. To extend the mandate further down the income scale, I would beef up and extend the credit to those not eligible. This method of denying public benefits is more easily enforced than trying to run around and knock on doors to collect sizable sums of money.

(See Gene Steuerle, Clinton Versus Obama on Health Mandates.)