The Klipsch Heresy's that silversport uses and the NHT T5's that I use will make a good example of a speaker's power rating.
First the Klipsch:
Efficiency = 97db/W
Max SPL = 114db
Max power = 100W RMS
Now the NHT:
Efficiency = 85db/W
Max SPL = 114db
Max power = 250W RMS
Looking at these two speakers shows that the Klipsch can achieve ~112db with ~64 watts. The NHT's can achieve ~112db with ~512 watts. For even greater efficiency soundhounds Altec's can achieve ~112db with ~4 watts.
Those numbers show why some need 100W amps, some need 200W and some need 8W.