Samsung had a little pressure sensitive area at the bottom of phone screens in the S9 era. It was handy and I had it set up like a hidden home button.
The S9 also had a variable aperture camera, which I feel like could have been developed a lot more.
I also miss notification LEDs. Sure, AOD mostly replaces it, but it was fun setting different colors for people, situations, etc
The Essential phone had a ceramic back that was very comfortable and cool in the hand. Though it was slippery. Another idea that could have been developed further.
The different back materials on the Moto X were very satisfying. The customization of that phone was unmatched!
That's honestly what I find interesting about it. The kernel of truth getting shouted out by the well deserved derision