Specific aspects of the Z2 that I really enjoy more than on the Z1 are
1. The screen is noticeably better, particularly from off angles
2. There's no longer a flap over the headphone jack
3. I prefer the wireless charging set up on the Z2. You don't need a stand in order to wirelessly charge.
I was also happy to make the change for a reason particular to me: my old Z1 had only 16GB - I had not realized when I bought it that you CANNOT install apps on an external microsd. So when I got the Z2 I made sure to get the 32GB version.