Nigel McFarlane writes an excellent article on the promise of Mozilla as a platform. Everything that he says in the article is at least in part attributable to Microsoft's XAML as well…except, of course, that MS' solution is non-portable and requires an entire operating system to deploy onto, not just a browser.
This talks to a number of important factors about the way that software development is changing…
You have the ability to target a platform, without having to worry about the complexities of the underlying operating systems or architectures. As mobile solutions get faster hardware (smartphones, etc.), we'll see these meta-operating systems extending there as well. Sure, you may have to design an alternate UI layout, but you could potentially re-use large chunks.
Another great quote to end off (links added by me):
Mozilla's XUL has been so successful and so influential that most vendors of GUI development systems, like Microsoft and Macromedia, now offer an XUL clone or an XUL alternative of their own -- XAML and Flex, respectively. One day, all but the most specialized applications will have their visual user interfaces specified this way. It's the future.