Windows: Over-commercialization (account requirement, OneDrive upsells, ads in Start menu, Copilots everywhere). Runner up: bad performance.

Linux: Absence of a strong, universally recommendable distro. Ubuntu pushes Snap which has all kinds of problems. Fedora doesn't include proprietary drivers, causing problems with GPUs. PopOS is in the middle of switching DEs. Arch is Arch.

macOS: Liquid glass.