At it's most simplistic a really great hifi doesn't sound like a hifi at all. It doesn't sound like a collection of boxes and transducers that produce "as close as possible" facsimile of events that have been recorded for reproduction. This has always been my aim.
As Bob noted, a sense of ease which is what many systems lack. This relates to the ability to do scale, dynamics (micro and macro) without strain and allows you relax into the music.
Resolution and reproduction of low-level detail preserves all the cues and details that recreate the ambience of the studio / concert hall, and bring the event into the room.
A property I listen for is "energy" Which I think this relates to micro/macro dynamic range. This may be treading perilously close to the PRAT thread / car crash.
Natural timbre and neutrality are preferred (though IMO not the be-all and end-all) but very obvious discrepancies will eventually jar and distract from the illusion of music in the room.
The better a hifi system is, the less you notice it.
My question was because the reasoning about greatness I sometimes hear is fundamentally "I know it when I hear it". Without details, IMHO that isn't really informative. Hence my interest in some more precision.
I don't get your use of "energy" at all - perhaps that's my misunderstanding.
I don't agree exactly about "
as close as possible" to an event - my version of that is "
not in contradiction" to my experience of many similar real-world events. That is, to me anyway, rather different and leaves a quite wide latitude in my definition of greatness. That will also be personal in detail, since my real-world experience will mostly not be the same as for anyone else. So IMHO greatness almost certainly is a personal matter rather than a universal one.
Timbre/tonality. Yes, this needs to be plausible for my key content: voices and piano to start with. Piano needs frequency response down to about the bottom note on an 88-key piano: 27.5 Hz. I am not in the "20 Hz" brigade and I am not sure there's much content in my normal listening in that lowest half-octave.
But that 27.5 Hz needs to be reproduced at rather high SPL when demanded. Too incapable a bass driver and to my ears tonality changes displeasingly with dynamics and instrumental independence suffers from blurring. As volume goes up (including on transients) I recognize the need for a "sense of ease". There are candidate technical explanations I have for these but I won't go there now. And there is more in my mind about these and other issues but perhaps that's for later.
My experience (from shows only, though) is that very expensive, very high-end systems can still do things that displease me by contradicting my experience. And less expensive systems can please me greatly within quite a latitude by doing nothing wrong within their capabilities. By not sounding like HiFi I personally think both ends of the range can achieve greatness.