Miracast support is one of the reasons I stick with Samsung. Google stripped it out in Android 6 but Samsung kept it because their smart TVs use Miracast. I use it on a regular basis and unlike Chromecast it's a universal standard. Being able to cast my screen to any smart TV I've encountered, Roku and other streaming devices, and even windows PCs is incredibly valuable to me.
Themes were big for me as well, but I believe stock Android has a dark mode which basically covers that for me.
Stock Android has come a long way, many of the features that kept me on Samsung are now available in stock Android. Split screen was big for me at one time.
At this point it's the S Pen, native miracast support, and the camera hardware/software that keeps me buying Samsung Note phones. DeX and some other features look interesting, but I don't use them.