I don't think there is one thing. One of the things that is quite interesting is there are all these different people and different companies and they all have their own agendas. For example within the mobile world - which is not just one area either - there are the cell phone companies, the embedded systems, people using Linux for things like embedded music, distribution points like wireless music and media in general, and they all have their own thing that they think is the most important, which is true for them because they may have a small or even a large company that is going in one direction.
So what motivates you to work on Linux?
Linux has done what I wanted it to do for the last fifteen years, literally, so since very early on my motivation actually came from the outside: my motivation came from problems that other people see. I don't see the problems, my usage model is actually fairly simple and it's still the same development model that I tend to concentrate on. So I am actually motivated by other people's issues, and sometimes they make me go "those people are just crazy, that's just insane", but sometimes the crazy people have some of the most interesting problems too.
I personally have always been interested in the desktop because my usage has always been as a workstation. It's been a fairly limited desktop because I don't tend to care about a lot of the things that other people care about on their desktops, so personally I tend to be more intrigued by problems that other desktop people have as it's closer to my own usage.
But at the same time, sometimes the really technically interesting problems in particular come from the embedded world. They come from people who have very strict requirements because they are doing just one thing and they are usually doing it on a very small machine because they need that machine to be cheap in order to sell millions and millions of copies. So sometimes the desktop people have a much easier time because their machines are really big and powerful, but the really interesting technical challenges sometimes come from the mobile people. Although for them it's usually the user interface, they don't tend to complain that much about the kernel.