Great answers all! Here is what I was thinking when I asked the question.
I realize everyone is different and tastes vary greatly, but if we could boil this frog down to a single word that encompasses the essence of audiophile nirvana I'd have to select REALISM. If the speaker conveys realism the system should disappear allowing the performance to take place in our ears and mind. If we are presented with realism the music should engulf (engulf = sound stage, presentation, dynamics, extension, SPL levels, accuracy, air etc.) us and allow for nuances previously unrealized. Realism allows the vocals to be presented exactly as the singer intended. Realism gives a seat at the concert where we can hear the plastic pick brush the guitar strings and we feel the air from the kick drum. Realism should never be fatiguing because everything is in balance. As we sit in the demo chair we are presented with a technical gateway between the recording studio/concert hall and our ears, but this conduit should never color. It transports sound, but never gets in the way - REALISM.
Lastly, a good speaker should play all music adroitly. A speaker that plays classical music with aplomb, but can't handle rock or jazz won't get my attention.