Slashdot pointed me to this ZDNet blog entry talking about Rich Internet Applications. In there is a paragraph on XUL. I quote:
While I’m sure I might get flamed a bit by the XUL community for saying so, XUL is currently a good working model for how not to build a broadly accepted RIA platform, despite some technical greatness.
Believe it or not, I actually agree. It is Mozilla-only, and Mozilla-based products don’t make up a massive part of the market share. (Fifteen percent is respectable, just not massive.) XUL would be much better if it was a standard implemented for multiple platforms – but each vendor makes their own decisions on what to do. This isn’t necessarily a bad thing, but it’s something to deal with.
If the W3C were to provide such a unified language (and with XAML coming up, it may be forced to), then all would be well. But I don’t expect W3C to do so. Maybe WHATWG… maybe.