Init managers for sure! Amongst file managers and DEs, firewalls, package managers, modern packaging systems and their sandbox/security systems, display servers (probably the funniest one), audio servers, filesystems.

Lots of stuff we should appreciate having as FOSS, especially the options we don't choose.

Fully switching over for the last couple years has made this modularity feel especially apparent compared to commercial systems (when things aren't always so seamlessly integrated) but I'm glad for it all; it's really fucking cool to think about how dramatically you can change the experience of a Linux desktop OS.

Probably X vs Wayland. Everyone knows what the correct answer is.

I mean, it could be so many things. Could just be people fighting over distros in general, or it could be the wayland vs x11 thing.