If you happen to be the only one with negative interest rates, you also weaken your currency, which means you make your exports more competitive.

[When] the market is trying to get to terms with, first, lower global growth, particularly out of emerging markets and China. And, second, the market is worried the central banks have run out of ammunition. So put these two things together, and then investors are repricing the market lower.

Investors should be cautiously positioned as the global economy and markets face major uncertainties. The downgrade will be a further headwind to growth and job creation in the U.S.

The once-unthinkable loss of the AAA rating will constitute a further hit to already fragile business and consumer confidence.

We normally think if you're going to lend someone money, you should get some reward for doing this. In Europe, it's a tax!

America's downgrade may serve as a wakeup call for its policymakers. It is an unambiguous and loud signal of the country's eroding economic strength and global standing. It renders urgent the need to regain the initiative through better economic policymaking and more coherent governance.

The theory is that if you take interest rates negative, people are going to say, "That's a silly game! I'm not going to lend my money to governments who want me to pay them. I am going to go into the stock market where I can get positive returns!"