I'm not really bothered about custom ROM's, but basic functionality. I should be able to plug a phone into a windows or linux computer and transfer what I want, without additional software / hardware. I also hate the whole emphasis of going through the cloud. For personal data, I'm not comfortable with this.
If I'm honest, I'm not really familiar with the whole limiations of the iPhone, or whether things have improved or got worse over years? I know when I looked into it years ago, I could not even plug an iPhone into a windows or linux computer and copy audio files (mp3, flac) into folders of my choosing. It seems people were forced to go through crapware like itunes (which did not even have an official linux release). Not sure if any of these restrictions still apply?