A few basic things which come to mind:
Resource management sucks under pressure. It's still possible in the present year to lock up a typical Linux desktop (running with default settings) by copying large amounts of data to/from slow USB drives. I shouldn't have to ionice when using cp to stop things hanging. Likewise for memory management when under extreme memory pressure, the system has no idea what needs preserving to maintain an interactive desktop and what doesn't. You can stress test Windows and macOS to the extreme and not end up with a completely unusable system (with Testlimit and memory_pressure, respectively) without the need to cgroup everything.
Audio DSPs are still poorly supported, with ALSA drivers unable to program them to produce the same high quality audio that OEM-provided Windows and [proprietary out-of-tree] ChromeOS drivers do. Easyeffects is not a fix for this, as it does everything in software. This means if you have a laptop with built-in speakers, they are almost certainly guaranteed to produce inferior sound on a mainstream Linux distro and desktops with built-in sound cards often can't play audio through wired external speakers as loudly (without distortion).
Discrete GPU support also is still lacking. NVIDIA drivers have no support for offloading to shared memory at all, while AMD drivers have power management issues (especially when using high refresh screens) using a bunch of workarounds to handle low-memory scenarios in lieu of proper VRAM management. Said workarounds (for example, offloading inactive VRAM to system RAM aggressively but rarely re-promoting back) will be just fine for the upcoming Steam Machine, where the workload consists mostly of the Steam client, a custom compositor and a video game... but will still remain inferior to how Windows manages it.
Hardware accelerated encoding (especially for WebRTC) is still pretty much non-existent across web browsers. Google locks it behind ChromeOS for Chromium-based browsers and Firefox is dependent upon a pair of Red Hat developers contributing gradual changes out the kindness of their heart. Thus, functionality like group video calls runs like absolute ass on mainstream FOSS Linux distributions when compared to ChromeOS, Android, iOS, macOS and Windows. Worse still, for NVIDIA and AMD users, even decoding is still unreliable, barely supported or outright broken.. and even on Intel GPUs, anything dependent upon Widevine won't hardware decode (again, unless you're specifically running ChromeOS). Even software like Discord struggles, and this is with Steam Machines on the horizon.
TL;DR: Having hundreds of distros, tons of desktop environments etc. isn't a problem. The technical underpinnings sucking *is* a problem.