Modifyability is what’s most important to me.
Even though Android is open source, to me it doesn’t feel open source because it’s not designed to be a good development environment and forcing me to re-flash the device if I want to modify the OS. The fact that OEMs pre-installed ROMs are closed-source also means that there will never be easy to integrate and distribute changes to the OS. Android doesn’t also give a shit about being modular and has fewer CLI applications which are very important when you want to know something about your system. Often if you want to get some deep information about Android OS you need to develop an app in Java because it’s only easily accesible from the Android SDK which is very annoying.
Security and privacy is also nice, but I buy this device for the freedom.