What is a Good Speaker?

Hmm..Debatable question this Doc !
The speaker does have the most impact on the sound but still its qualities should not be taken in isolation as it is its interaction with the Amp , the source as well as the room which determines its qualities

But some well agreed highlights of a good speaker would be

1. Clean midrange ie voice not sounding "Boxy"
2. Cleanest reproduction of sound, for each instrument
3. No Boomy bass (Like the 1200PMPO stuff !!)
4. Clean high frequecies..no shrill sounds..Asha Bhonsle should not make you cover your ears
5. Good soundstaging ie once the toe in is correct, you should be able to image the singer in the middle and each instrument in the music around the singer and aslo be able to identify each instrument
6. A guitar should sound just like one and not like a sitar or a Banjo.

Well..

to put it in a nutshell :

1) good speakers should sound as natural as possible. (All the points that ARJ mentioned hold true here)
2) compare the sound of your speakers to what you would hear if the music were played in frot of you live.
3) Should reproduce the timbre of the instruments correctly
4) Full or extended range speakers are not always the best. Some speakers go down to only 45hz but they play most of the music 45hz and above very accurately.
5) Flat response speakers are generally termed to be good but may not be the case if they dont measure up as far as transient response is concerned.
5) Over and above all of this, a good speaker is what sounds good to you !