What's the most surprising thing you have learned about the piano?

How well you practice and play a piece at home is in an inverse relationship to how well you perform it for your piano teacher.

The people that restored my grand piano said it would take around 3 years for the instrument to come into its own. It really did. And that even a new piano takes time and effort to tweak into becoming its best self. For my space this meant even replacing all the hammers, ultimately starting with a completely "unjucied" version. And that voicing hammers is kind of a one way street. You can needle them to soften, but they will harden over time. If you apply hardeners that is not reversible. So starting soft and very, very slowly building up seemed a good way to go. 

I have a friend in Paris with a tiny 4' Playel grand from the 1960s. I love this very gentle piano, but It needed some work. Was very surprised that the action is plastic. 

Related to what Dominic stated, I am most surprised by the anatomy of the Piano! I so enjoyed the Tonebase Lesson Dominic presented where Neema Pazargad, Director of Piano Technology, explored the inside of the Piano and told us about how it works. Fascinating!