Water, below freezing point (0 degrees C) can still remain in a liquid form until it hits an object. This object creates a shock which allows the water to solidify. Hence ice on blades and fuselage = out of balance and extra weight.
A lot of investigation into the formation of ice has taken place. The result is that modern aircraft have ice sensors which uses optical technology to measure the impacted ice build up on a bar and indicate to the pilot the liquid water content based on timed readings.
Heated rotor blades are used on some aircraft, these use masses of electrical power and are not normally that reliable due to the harse enivironment.