To me, I think it is very important to look for very efficient speakers. This not only allows to drive them with less power and as a result might help to reduce the overall cost of the system. If you drive these with amplifiers that have plenty of power this allows the sound to be transferred to the speaker with ease. This extra power also allows for greater head room and in turn might reduce distortion. I equate this to driving car with 200 HP and another with 400 HP at 100 mph. The 400 HP car is going to support a speed of 100 mph with ease. If you were to insert the word resistance into the equation, a speaker with less resistance will have to work less and the less it has to work the lower the distortion. However, the next thing one must consider is the speed by which the speakers can react to the signals it is given. However, all of this will affect how different styles of music sound better as well. We might think then of the speaker as a musical instrument to produce the right amount of resonance desired for the styles of music played.
I am a rookie and certainly I do not know what the rest of us knows. I am just trying to mentally figure this out. I also agree it is important to take into account the size of the room, along with things like the amount of hardwood floors, rugs and furniture to absorb the right amount of sound.
I kown Carbon Miller might have the correct answer to this question.