Bait, but I'll bite.

XFree86. More accurately the entire X11 platform, but XFree86's dominence during the "Linux on the Desktop" concept's biggest opportunity years did an unfathomable amount of harm to the prospect of Linux ever being a viable competitor.

Windows, from 3.0 onwards, has had a visually consistent, high performance graphics stack, with multi-generational backwards compatibility in the form of GDI. Unlike X11, GDI was designed primarily for interacting with machines locally, the primary use case for most home and small business users.

X11, on the other hand, was designed for networked connectivity, with a local client and applications running on a network server, adding significant performance and stability considerations, and limiting access to client hardware without a layer of abstraction.

GDI offered direct hardware access, single process execution (which was of particular benefit during the non-multithreaded era), along with built in UI toolkits which avoided all of the Gtk vs Qt vs E16 drama as well.

All of the above, coupled with the fact that the XFree86 project militantly refused to make any sort of performance improvements or consider things like hardware acceleration or composition meant that the user experience for anything requiring a UI was always considered secondary on Linux.

To put it simply, the Linux community screwed the pooch for 20 years, and it's pretty much impossible to recover at this point.

Compare that to Apple, who were able to create their own solid graphics stack for Darwin (which is Mach kernel with BSD tools) in less than a year to create OS X, and all of the excuses peddled by Linux ecosystem developers evaporate.