Tyler Cowen warns:

The differential payment rates across Medicare, Medicaid, and private insurance are becoming unsustainable more quickly than I had anticipated ... Further reforms will be required more quickly than had been anticipated, but it's not obvious how such reforms should proceed.  It's hard to either upgrade the Medicaid (and Medicare) rates or to downgrade the private insurance rates.  Monitor this one closely, because it is likely to prove the breaking point of our health care status quo, with or without the Obama plan. 

Ezra Klein explains the issue in more detail:

The problem is that Medicare can't control costs too much better than private insurers or ... doctors will simply abandon Medicare. In a world where there's only Medicare and Medicare decides to control costs, doctors can either take the pay cut or stop being doctors. And as we see from other countries, lots of people want to be doctors, even if being a doctor doesn't make you particularly wealthy. But in a world where Medicare is just one of many payers and Medicare decides to control costs, doctors can simply stop taking Medicare patients and a lot of legislators will lose their jobs. 

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