input sensitivity

Input sensitivity is the level needed to drive the amp to full power. It is measured as a voltage. For example, an amp with 500mV input senstivity will only need 1/4 the output of a normal 2V CD player to reach full power! This means an active preamp would be unnecessary.

High input senstivity (which means low input voltage needed) will make you run out of volume control range - which is very annoying. You will have to barely turn the volume control to make it loud. However I have come to an informal conclusion that high senstivity amps have a very full bodied sound with great authority. Not sure if I am imagining this or not but Jadis amps are good examples.

Arthur

Another thing you should be concerned with is input impediance, can make A big difference in overall sound more then most people want to think.

An amp with high sensitivity is more prone to noise pickup in the interconnect. I prefer to boost the signals to a good high level before sending them through my (admittedly long) interconnects. However, with good wire and good electronics noise should not be an issue, but the oversensitive volume control, mentioned by Aball is a problem.

in addition amps with high input sensitivity have limited dynamic headroom.
powerfull amps usually need >2V input voltage to reach full output power.