This move has severe philosophical implications. Negative interest rates magnify the time value of money by turning cash into a wasting asset. Combined with inflation, which has been a nonzero value for most of Europe's postwar history, negative rates accelerate the debasement of consumers' purchasing power. Europeans are now forced to immediately spend their earnings or invest them in assets whose risk profiles lie outside what would normally be their portfolio's efficient frontier. The ECB has just laid waste to modern portfolio theory.
The ECB has stepped into darkness and the world's financial markets are not registering an iota of care. US equity indexes are at record highs and the CBOE VIX volatility measure is comatose. Investors worldwide are lounging blissfully unaware of this decision's risks. Negative rates on bank reserves will force banks to lend cheaply. If they're borrowing short, they risk exposure to an ECB policy reversal or a surprise drop in the euro's value. The cheap lending will accelerate monetary velocity, which will massively magnify any rise in Europe's rock-bottom inflation rate. Europe has painted itself into a corner. It will make a mess on the way out.