One thing I haven't seen mentioned: how do you design an organization that manages and efficiently allocates $3-4 trillion in spending across the country? Has it been done before? If so, has such an organization been legislated into existence? Meanwhile, Aetna already insures more people than many countries, so it isn't clear what we'd be getting that we aren't already. I'm curious how single payer will help in the US.

Medicare is actually relatively efficient compared to private insurance companies, and administers a very large amount of healthcare reimbursement.

Insurance is one of the things that a government can be very good at. It's not an area of great innovation, so the benefit of having it be a free market is relatively limited, and negotiation leverage is very important, so size is very beneficial. The natural tendency of the private insurers has been to consolidate and to look more and more like Medicare in terms of size. Except they also have a profit mandate, whereas Medicare doesn't, so they take a percentage of all spending and suck it out of the system. That portion isn't helpful to the users of the system.