Thomas Klitgaard and Harry Wheeler write,
The discussion above offers up a perspective on what is meant by “monetizing debt.” This term refers to a central bank buying government bonds and promising to keep them on its balance sheet with the result that the increase in reserves in the banking system translates into higher prices. This outcome, though, requires that the central bank not pay the appropriate interest rates on reserves. If it does, then an asset purchase program is just an effort that shortens the maturity of public-sector debt and will likely have few or no implications for future inflation.
Pointer from Mark Thoma.
Another implication is that it makes the interest cost of the government more sensitive to movements in short-term interest rates. So a sudden loss of confidence in the government by investors which raises interest rates would become self-reinforcing. And if the only way out of such a debt crisis is to print money, then there are implications for future inflation.