Money transfers between bank accounts for no extra fees (well, that's limited to the Euro, so a couple of countries chose to stay outside). Mobile phone usage for no extra fees all over EU (some limitations, but typically not relevant for the average trip).

B2B the integration has even bigger impact.

I don't think eletric plugs are even regulated. British plugs never changed and they were not an argument in Brexit.

USB-C!

How about tap water has to be drinkable in every EU country