For example - if we were to go to a purely government single payer system, what are the cons? As we can see with California Medicaid, the VA system (funded by Congress) - when the government needs money, healthcare is on the chopping block. With single payer, you have no leverage, no other source of income. You will take what the government pays you. You see some of this right now with medicaid/medicare providers, especially those whose patients population are mostly medicaid/medicare - they are affected by state government budget as well as the medicare budget (with the annual threaten cuts). Tackle on this $700 billion-$1 trillion bailout, Congress may just cut reimbursement to help balanced the budget. Remember it was only recently that Congress got rid of the economic hardship deferrment for medical residents to help with the budget.
Also associated with government payment system are insane rules. Right now you cannot balance-bill. If the actual physical cost of a patient visit is $100 (rent, utilities, office staff, insurance, etc, excluding your income) BUT medicare only pays $70 - you cannot bill the patient for the difference. You will have to see that patient for $70 and take a $30 loss on your books.