How does solo piano help you evaluate audio gear?

Good points and very helpful. I went out of my way *not* to ask if it should be the only one. I'm really drilling here for phenomenological details, i.e., descriptions of what one notices in a piano recording that has probative value for a more general conclusion about the capabilities of the system. There is some of that in this post by McGowan.

Acoustic instruments go a long way in the realm of realism when a system is firing on all cylinders. If the initial movement (of a key, valve, etc.) is captured, followed by the strike of it, followed by the burst and then bloom, and then the associated spread, decay and reverb, the whole soundstage develops to a degree that can make one momentarily forget they're listening to a recording. A piano is a great instrument that can do that.

One can faithfully replicate the passing of a heavily laden semi without all the shaking when a piano plumbs its depths. A microphone placed close to the soundboard can be all it takes to make you forget and just revel in it all.

If your system can already capture and recreate that kind of moment, then you're almost there.

All the best,
Nonoise

The sound of a piano seems to be one of the easiest instruments to recall.

In playback it also seems to be one of the easiest ones to detect any sonic issues.

As said earlier, it’s an issue of power, weight, and scale.

Perhaps tone and reverb too as no two ever seem to sound the same, and you don’t need to be Glenn Gould to feel that way.

reproducing piano is the toughest test for any music equipment.

it has to sound clean throughout all dynamic passages and throughout the entire keyboard. 

why it's the toughest test, because piano has all instruments combined. when you hit the string, you literally play drum and note together.

I've owned and played my share of pianos over the years, but I also play the fiddle and I have to say that massed strings is the true litmus test for determining the ultimate veracity of a system. And when you're judging a system by a piano recording, don't get too crazy! Many fine pianos sport a fuzzy tone in the midrange upwards, so you got to take what can sound like a mistracking cartridge or a gently clipping amplifier section in stride. And of course, as mentioned above, reproduction of the human voice shouldn't be given short shrift, either. I mean, how often do we hear a live, un-amplified human voice? A zillion times a day? True, it's not often enough that a vocal is recorded with true, convincing accuracy, but believable vocal rendition is still a worthy audio goal. In any event, true, dynamic range is important but I'll take a slight dynamic hit every time to preserve tonal quality and lack of distortion.