1. The wireless charging, while seemingly a small thing, was to me a huge improvement in my experience of using the phone. The fact that I can sit down at my desk, plop the phone down, and it'll just work, and when I get up to leave I can just pull it off my desk. Removing the USB cable interaction made the phone a much more invisible and organic part of my day. Might not be important to others, but I would very much miss it.
2. Lack of removable storage. Small deal but it was a nice quality of life feature.
3. Battery that's hard to replace. My phone upgrade cycle is very long, usually longer than the lifetime of the battery. I'm feeling it in my Nexus 5 right now.
But the main thing that might prompt me to upgrade is just software performance creep. The latest software updates have really turned my phone into a slideshow, and that's getting really frustrating.
But the spec improvements are certainly both appreciated and necessary.