Mostly the prebuilt binary packages, "casks". Out of 13 casks I currently have installed, only 6 are in nixpkgs. Some are built for Linux (e.g. Ungoogled Chromium), but not for Mac.

I've also cherry-picked a couple non-cask packages that I would dearly miss: dark-notify, ddcctl, displayplacer, the kinda stuff you need to "fix" macOS.