Nothing

If you are using Gnome distros: you can feel exactly what it feels like getting back to working in a restricted, overhyped, overbranded environment like Windows.

If you are using Ubuntu: you can get advertising during your system’s software upgrades. No, really.

If you are using Arch: you can post aroudn the internet saying you use Arch btw.

Depending on the distro, you can use some alternative software stacks, but that’s mostly the backend (eg.: systemd versus openRC, Apache vs Nginx, X vs Wayland); most “desktop app” level is mostly the same for each desktop environment, is kinda the point.

Technically speaking: nothing really, provided you have time and skills.

Except maybe not having access to NDA-ed binary blobs or something…

Compared to Arch(-based): Accesing the latest packages. It’s not impossible, especially if you go for Debian testing repos, but it’s definitely extra work.

Compared to special-purpose distros (i.e. gaming, portable, high security/privacy, pen-testing): Whatever their special purpose is will usually be harder to achieve.

Compared to huge corpo distros (SUSE/Fedora and derivatives): Ease of more intricate setups and maybe some security testing.

Compared to Ubuntu: Paying a corporation to not withhold security patches from you.

File-by-file integrity check against signed checksums upstream to trivially confirm validity of deployment.

But that’s probably not interesting.