Spaces, the windows 10 version won't fill that gap for me until I can assign apps to specific spaces so that they always open there, as well as key shortcuts e.g. Ctrl 1 to go to space 1. Also remembering my setup through shutdowns.

Quick look - so useful.

Screenshotting (cmd shift 4, cmd shift space etc) and annotations via preview app. Windows expects me to draw highlight boxes by hand which rapidly looks unprofessional.

Using cmd shift 4 to measure pixels for web design work.

Exposé is nice, not sure if Windows has something like that, I think it does?

Spotlight was a big one for me too but I believe Windows does that justice now with Windows button search.

iMessage on desktop is actually a big deal for me, being that most of my friends and family have apple devices. Being on Windows and wanting to send someone a quick link is frustrating when you're used to having iMessage.

As others have mentioned, it's all the small things. Admittedly I haven't looked at Linux on the desktop for a while so this is mostly based on comparisons with Windows.