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

Everything about it is so complex. The unthinkable things we have to keep track of simultaneously... both hands playing together while reading different cleffs which have the same lines and figures but mean different things, doing different things in different dynamics and pedal and so on... It amazes me and it overwhelms me... I feel like an idiot most of the times... I feel I need two brains, one focused on each hand, to maybe one day be able to play like I would like to play. The only thing that keeps me from giving up on myself (besides the beauty of the instrument and the feeling of finally doing something that felt impossible in the beggining) is knowing that many other human beings can play it well enough and that I can not be that dumb to not be able to, one day, feel like I can, at least, play at some acceptable level. I've only been learning for a bit more than 1 year now... so I know it will take long... but I really want to get there. 

The hight of key, measured from floor level, differs from piano to piano. I switched from Yamaha upright to Schimmel upright and then Grotrian-Steinweg. They are all quite different and need to adjust how I like to sit. I went to store and beg them to cut legs of bench 4 cm shorter with Schimmel upright. Grotrian is taller (meaning more space from floor to keyboard).

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!

All the colors that come from the piano.  

Many years ago, I was surprised to learn that the piano is considered a percussion instrument!

Use of the middle pedal. 

I'm always surprised at how completely unforgiving she is when I've ignored her for a few days. 

Apply the music theory for analysis the pieces and playing piano with mechanical aspect.