A lot of central banks have moved towards negative interest rates. Some people think it is essential that central banks ease below the zero boundary. This includes central bankers at the ECB, in Denmark, Sweden, Switzerland and Japan who have all implemented negative rates.
Some critics believe that negative rates will have all sorts of unintended consequences. Even proponents of negative rates sometimes lobby for a move towards a cashless society where negative rates can be implemented with potentially fewer unintended consequences.
Regardless of whether side is correct, negative nominal rates are a sign that central bankers will fail to accomplish their goals.
In the case of critics of negative nominal rates being correct, the reasons are relatively straightforward. Something will go wrong. Maybe attempts to avoid negative rates by changing deposits to cash mean negative rates merely incentivize unproductive behavior. Perhaps negative interest income of banks drive deleveraging, or the uncertainty of negative nominal rates negatively impact the markets. There are many possible unintended consequences that could cause negative nominal interest rates to be counterproductive.
What is interesting is that even if negative nominal rates are necessary and proper, things will still turn out badly in our current environment. Central banks are naturally cautious, and negative interest rates a new territory that they will not dive into. When central banks think an interest rate cut is necessary in positive nominal rate regimes, they usually move in increments in 25 or 50 basis points. However, many central banks are being very reticent about moving interest rates negative around or past the zero-bound - the Bank of Japan is a good example. They only moved the rate to negative 0.1%, and only apply it to specific accounts held by financial institutions held at the BOJ. To the extent that negative rates are necessary and proper, central bankers are too scared to move the rates sufficiently negative.
So either they are messing things up by implementing negative rates, or their negative rates won't be sufficient to accomplish their goals. This helps explain why the Japanese yen has actually strengthened since the implementation of negative rates by the Bank of Japan. Either they aren't doing enough, or what they are doing is counterproductive. Even if it is difficult to parse what negative nominal rates will do, it is safe to say that central banks resorting to the implementation negative nominal rates are unlikely to accomplish their goals.