The overall sense of control I have over the system. With iOS it feels like I am using someone else's phone, it's not as customizable/optimizable. I can't install other apps on it (outside the store that is) and the entire dev process feels so..gated. I want my phone to be an extension of my tech environment, something I can automate and freely fiddle with.

It might sound petty but having to jump through hoops to use a custom ringtone was the nail in the coffin.