I think everyone can agree that Linux's biggest shortcomings on desktops/laptops are the graphical and audio part. So, even though I agree with "As soon as someone is willing and able to put in the effort on desktop Linux, it will be as good as it is in those other areas." I would replace "someone" with "every vendor".

I feel like most vendors (NVIDIA/ATI/Wacom/whatnot) concentrate much more of their effort into supporting Windows and even OSX becasuse thay's their audience.

Also, I remember reading somewhere that NVIDIA/ATI work closely together with Microsoft because od Direct X[citation needed, though]. I had the opportunity to work with Direct X (the new API) and I found it much more pleasant than working with OpenGL (even though I ended up using OGL in the end; I used Windows and DX to simplify prototyping, because doing the same thing in OGL required much more dev time, at first atleast).

EDIT: Also, let's not forget how most of the majority of linux developer community neglects GUI and the overall end-user friendliness, and how the environment is in most cases quite hostile towards UX/GUI designers in general. There are, of course, exceptions, but those are few.