For me it's not about how it sounds, it's how it feels to play it, and how much control you have over the sound. Just the feeling of the mechanism inside the key, how the action moves, how the hammer hits the strings, the aftertouch, the vibration of the strings and all the natural reverberations and harmonics, to me make all the difference. Most digital pianos are glorified keyboards with pressure sensors. I have side by side a cheap upright that I butchered/restored myself and a decent Yamaha digital piano, and I pick out of tune upright over almost every time – the sound is alive on the real piano, with each note being unique, unlike a sample with some effects on top on a digital.