It depends on the specific sort of motor, as well as the mechanical load connected to the motor. If you are asking about a common three phase induction motor with a constant torque load:
The RPM will drop, to about 5/6 of rated speed.
The horsepower capability will drop, to about 5/6 of rated output.
The operating voltage will drop, to about 5/6 of original rated operating voltage.
The _losses_ will remain roughly the same, but will probably drop slightly; since the output will be lower with the same losses this will mean lower efficiency.
Cooling air flow will drop. Winding temperature will probably remain similar, because of the slight reduction in losses.
Bearing wear will be reduced unless the new operating speed results in some sort of mechanical resonance.
For many common mechanical _loads_, eg. pumps or fans, the load torque will drop substantially with reduced speed. The reduced torque should be factored in to any estimate of power required or reliability.
-Jon
