The first reasons are quite possible, and I hadn't taken them into consideration. They would tend to explain why even Windows OpenSource apps are ugly (see The Gimp) since they're frequently just ports of Linux apps. It would also explain why even freeware and shareware written specifically for the Mac tend to be very elegant and polished, Windows a bit less so, and Linux least of all (although I'll still maintain there's a mindset difference as well).
The last reason I don't buy, at least not for most OS apps. I'll grant it for the apps coded by one guy in his bedroom, but not for the apps coded collectively by a large community, at least some of whom would seem to be artistically inclined (again, see The Gimp). That not one volunteer, even the artists, would take some time to design a nice UI speaks, to me at least, more about community personalities than any sort of time or training deficit.
I'll use me and my brother as an example. I tend to have a shaggy lawn, but it's because I'm lazy, not because I don't appreciate a nice garden. I plant roses and tomatoes, and take adequate but not intensive care of them. Things thrive and taste good (the tomatoes, not the roses), but they'll never win a beauty prize. I have, however, had some ambitious seasons where I took extra pains to make things look nice, and I love a nice stroll through (someone else's) beautiful garden. My brother, on the other hand, derides people with nice lawns and gardens as wasteful of money and time, and will audibly snort if you ask him to tour a garden. I don't think it's a coincidence that I'm a Windows user and he's a Linux fan.