From the posts so far, there seem to be two separate schools of thought on how to respond:
a) By the way various key instruments are reproduced
b) By non-specific characteristics such detail, tonal accuracy, etc.
Personally, I believe that a) could be seen as a break-down of b) - with specific examples from a) used to illustrate points raised under b).
Having said that, I always add a personal aspect to the equation - how do I react to the music being played?
So, my list?
a) Does what is being played sound more like "music" and less like "hifi"?
b) Do I find myself getting "involved" in the performance (toe-tapping, humming, etc.)?
c) If I sit down with a book while something is playing, does the portrayal make me put down the book and listen?
d) Does the "sonic picture" presented provide adequate spatial information (imaging, soundstaging)?
e) Do instruments and voices sound "real" and not "contrived"
If all five answers come up positive then you can bet your bottom dollar that the following will also be true:
- Tonal accuracy is in place
- Dynamics (particularly on symphonic works) will also be in place
- Detail retrieval will also be there in spades (critical to spatial cue reproduction)
- Driver control & integration (taut bass, open midrange, clean highs) will also be present
One other aspect is overall "balance" - across various musical genres - that ensures that one enjoys all aspects of one's music collection and not just one or two types.
Oops... Speaker choice? These:

Tannoy D700s