From what I've heard and read, from friends/family who work in hospitals, the biggest money-sink in healthcare is emergency medicine. The reason for that is pretty simple--the ER can't say no. However, a GP can.
People know they can get care for free at an ER. But they'd have to pay to get into a doctor's office for an appointment. So that runny nose and cough you've had all week? Why not call an ambulance and get a free ride to the hospital? They won't even make you pay for it, because that's what all the insured taxpayers are for.
It is not efficient to use emergency medical services for non-emergent care. But, how to fix it? I mean, the obvious solution to me is to allow ERs to turn away patients who don't need emergency care. If someone comes in at three in the afternoon with a sprained ankle, they can GTFO.
If any of that interests you, I found a couple blogs that talk about this intermittently...
http://scalpelorsword.blogspot.com/http://docsontheweb.blogspot.com/Supposedly, these guys are actual doctors, surgeons, etc.