Higher Performance.
A high-end screen either IPS or Amoled. (I prefer IPS for real color)
Quality of the design. Like, Corning Gorilla Glass 4 or Draco Glass anti shatter scratch resistant.
Higher pixel density because a 720p in a 6" is rather an unfortunate.
Backwards compatibility.
Reliable Browser. Better to be from Google Chrome if possible.
More wireless features NFC, advanced WiFi, Bluetooth, IR Blaster, GPS, etc.
Optimisation built-in apps. Like the browser, camera, etc.
More built-in storage at least 128GB UFS type.
Shared Type C USB connector.
Ability to recover it in case of softbrick. :v