Design. Design is what killed the linux desktop. It never had it. OS X has it. Even windows, crappy as it may be, has it.
Before I go on, let me say that Design is NOT "making it look pretty". In fact, thinking that this is what design is, is what leads many linux advocates to reject the needs of design.
Apple's work looks pretty-- because it is designed to function well.
Design is about usability and understanding the user and making an interface for the user that works well according to the users understanding, perspective and needs.
Design is an engineering discipline.
Seriously.
The Linux community hasn't had that, and I've seen many of them reject it. In fact, you can see it in the rejection of apple's patents. This is why they think that apple patents are not original is because they reject that any engineering went into them. But that's just one example. You see it all the time in lots of contexts. Look at the UIs of Linux... they didn't design one, they just copied windows.
Literal copying is about as far from design as you can get.
Sure, over the years, designers have taken cracks at bringing design to linux, including the work of Ubuntu, but it is rejected by the community.
Rejection of design is a cultural trait of the linux community. They reject it as a discipline, doesn't even see that it exists. (broadly speaking, of course.)
But as users, they have been influenced by it and many of them have switched to OS X because it is the best designed operating system.
And then they write long blog posts about how its wrong that OS X does things a certain way ... based on their lack of design perspective that would let them see why things should work that way.
Its ironic.
But its fine- if you want to run a linux desktop and don't value or care about design, more power to you. Won't ever fault someone for making that decision. We should all use the systems that we prefer.
But the culture that doesn't value design, and can't even see it as an engineering discipline, is going to have a great deal of trouble making something usable by the mainstream.