I miss the Mac trackpad and gestures. I miss the trackpad size and the surface itself. Wiring up libinput-gestures helps some things but you still don't get things like pinch to zoom in Chrome, for example, and the overall experience is not nearly as polished.
I miss Mac OS HiDPI support as well. Fractional scaling helps a bit, but switching from HiDPI to LoDPI and vice-versa (docking/undocking) can leave some apps scaled incorrectly, make a mess of your desktop, and I haven't been able to get a mixed configuration working where only my laptop monitor is scaled while my external display is not. I end up running this 1440p laptop screen at 1600x900 or 1920x1080 instead just to forgo the headache.
The VPN clients we use at work were much more pleasant to use on macOS. We have the option of using one of two different OpenVPN solutions as alternatives, and they're fine, but again, the polish isn't there (little things like automatically reconnecting to the VPN without my intervention would be nice).
I can't really think of anything else I'm really missing these days, though. Things I explicitly DON'T miss: Docker on Mac is a poor experience compared to natively running on Linux... Having a real Linux userspace by default is SO much more pleasant than dealing with Homebrew (which is great if you absolutely need it but I'm happier not having to deal with it at all anymore). I don't miss the Mac keyboard and touchbar. I don't miss troubleshooting things on macOS... it is incredibly opaque in comparison.
Overall, I don't think I'll be switching back any time soon, but would love to see more attention to the overall fit and finish as far as the desktop experience goes.