XAML As Documents
Although XAML seems to have resonated very much with the development community and there has been a lot of discussion about its relation to HTML and XUL, its ability to wire up .NET object graphs, the impact it will have on the development process by involving graphical designers in the development areas, and so on...
...there's another implication of XAML that I think that people have missed, and I would like to see more discussion of. XAML is actually another serialization format. Avalon provides the ability to serialize any element tree, actually any .NET object graph, into XAML, not just read from it.
If application developers start writing document-based applications that follow the Avalon model of deriving new classes from existing elements, or even simply reusing existing elements, and turning on some of Avalon's designer services, XAML could quickly become one of the most common file formats. Developers could simply reuse Avalon's serialization services rather than write their own. Thus, XAML files would not only describe user interfaces, but they can also describe full documents.
XAML could become a universal document format. Of course, any custom element would not be usable by an arbitrary application that doesn't have explicit support for that element.