I moved from HTC M8 to Z3 instead of M9 because superficially speaking, I hate that the HTC has an asymetrical screen. When in landscape mode, one side would be wider than the other and I hate that. One thing I liked about the Z3 is that it has SD card support. I am not particular with removable battery anymore as I have a power bank. Besides, I realized, though one has 2 batteries for their phone, how can one charge the spare battery while the other battery is inside the phone? There is no stand alone battery charger for phone batteries. So, having an extra battery is moot as one needs to drain one battery first before he/she can change the battery and charge it.
I also love the magnetic charging port. It charges the phone as quickly as using the USB port. And it won't damage the USB cover with the constant opening and closing of the flap. How I wish that there will come a time when we can also use the magnetic port to transfer data.
Being waterproof is also an added bonus. And did I mention that I want my screen to be symmetrical?