Mark Pilgrim got something off of his chest in XML on the Web Has Failed.
As always, what "failed" means is a tough one.
HTML has succeeded in that everyone uses it, but has failed in that noone does it right.
Mark talks about how noone is Doing the Right Thing with respect to XML. Are you using text/xml instead of application/xml? Are your header all setup correctly?
Most small coding houses couldn't give a monkeys. They just want to whip something out and be done with it. "Works for me on IE. Next!"
The bigger problem that I have seen is that i16n is hard. As noone does the right thing, it borders on imposible. Maybe this is a market thing. If the market said "you must put more energy into i16n" then maybe it would. As a Brit living in the US, I definitely know that most people here don't really care too much about other languages. "They all know English".
But then, a project will come up where you have to work with Japan, China, and europe, and then suddenly they care. But this is rare.
So the solution is easy. Everyone should just speak American/English and be done with it ;)