Note that Dr. Buiter states that the one impediment to negative interest rates is the existence of currency/cash. As such, he recommends abolishing all currencies with the possible exception of very small denomination bank notes that would be kept in circulation for "the poor and old".
Why is cash so detrimental to negative interest rates? If you had the option of depositing your cash into a bank and seeing it decline in value because the bank was charging you interest on your deposits, you would likely rather hold it as cash (at least to a certain point). As I noted above, a handful of nations in Europe are already facing that issue, however, at this point in time, their are not passing along the cost of negative interest rates to their customers with one exception, the Alternative Bank Schweiz, a small Swiss bank which is imposing rates of between -0.125 and -0.75 percent, depending on one's bank balance.