Two related but distinct concepts drive much of a piano's tonal quality, and both are more informative than simply counting visible cracks.
Crown
Crown is the slight upward arch built into a soundboard during manufacturing. This arch gives the soundboard the structural tension needed to vibrate efficiently and project sound. Over decades, humidity cycling and general aging can cause a soundboard to lose crown ("go flat"), which reduces tone and sustain even without any visible cracking.
Downbearing
Downbearing is the downward pressure the strings exert on the bridge (and through it, onto the crowned soundboard) due to the small angle change where strings pass over the bridge. Correct downbearing keeps the bridge firmly coupled to the soundboard for efficient energy transfer; too little downbearing (often from a flattened soundboard) results in a weak, buzzy tone even if the strings and hammers themselves are in good shape.
Neither of these is something a buyer can measure directly without training, but understanding them explains why a technician's assessment of soundboard condition looks well beyond just counting cracks - and why two pianos with a similar crack pattern can sound very different.
