Someone on QRZ.com asked today what "72" meant at the end of a CW contact. He thought they had sent "73" and he had simply mis-copied. But after the third time he realised that they were really sending "72" and he wanted to find out what it meant.
As a QRP enthusiast I soon realised that other QRP ops would send "72" instead of "73", so I guessed that it just meant "73 (best wishes) from a QRPer."
The principle of QRP is one of doing more with less. But Nick, KD5RFT spotted that it actually requires slightly more power to send "72" than "72" - one more dash and one less dot - and suggested that perhaps it would be more in the spirit of QRP to send "74" instead!