- Locks in fee-for-service medicine. Hard to change once implemented. Medicare's coverage of preventive services has been poor.
- We need a lot of innovation in payment and delivery services, and single-payer blocks that.
- Too much entanglement with politics. Think of how the earmarks will work
- Government can't set every price correctly. There are too many of them!
- Tax burden probably too high for the US.
- Government isn't really designed for efficient program management.
- There's little accountability for poorly run public programs.
- There's poor customer service.
- Legislators don't want efficiency.
- Medicare's low administration isn't merely efficiency, it's also undermanagement.
For obvious reasons, I'm skeptical of any argument against national healthcare that boils down to "government doesn't work." That's ideology, not argument, and this list is heavily padded to make the problems with single-payer look a lot worse than they are. I don't think you'd need to do that if you were making an honest case.
As Ezra says, this is all a bit academic anyway since neither single-payer nor single-payer-ish systems are politically feasible at the moment. But fair is fair. This is a list of four items, not ten.