For example, because I use OS X, I can go to any store, purchase my choice of Lexmark printer, and be on my merry way with a minimal amount of checking. The fact that I have to know that there's a "reference platform" is the problem. I don't want to have to think about that crap. I don't have to with my Mac. I want to save my mental cycles for important things, not struggling with which printer to purchase.
Or I can put it in simpler terms. There's a concrete social cost to using Linux on the Desktop. Let's say I want a printer for Christmas. If I'm running the amount of information I need to give my family in order for them to buy me one that works is orders of magnitude smaller than if I'm running Linux.
In fact, I'd sound like some sort of bizarro printer snob when I make my request. "If you're looking at Lexmark printers, these models work, these ones don't. HP is good, but not models like X, Y, and Z."
Now, like I said, it's been several years since I regularly used Linux on the desktop. I'm sure it's better. But the problem is not that there aren't known-good Linux configurations. It's that (1) those configurations are essentially arbitrary and therefore not obvious and (2) they require time and energy to learn (memorize, really).
If it's changed, that's great, but that's also part of the problem. Now I not only have to keep a set of known-good configurations in my head but a rolling changelog, too, to make sure my knowledge is up-to-date.
I switched from Linux to OS X precisely because I decided my mental energy was better spent on other things than remembering, "Oh yeah, the Dell Lenovo X876123 mark 10 is a known-good Linux laptop."
I'll add that I'm sure in aggregate Linux supports a much wider range of hardware than OS X. That's great! That's why Linux is kicking ass in all sorts of non-desktop markets like mobile devices, embedded systems, servers, etc. But there is an inherent tension in an OS that's optimizing for that vs. constraining itself to the desktop and the desktop only.