XML is almost 10 years old now. IT Conservative brings up something, albeit making it bigger than it really is, that is a point worth making:
I grew up with the idea that it is better to put a bit more effort into something at the beginning so that you would safe a lot of cost later. XML is the perfect example of something that does exactly the opposite. XML is simple. It's simple to write, it's simple to program with, but it is very verbose. Simple things take kilobytes of space. Why is that bad? Because it takes memory, processing power and bandwidth. You only have to program something once, but the memory usage, waste of CPU cycles and waste of bandwidth just keeps going on and on and on forever.
I know that binary XML has been brought up over and over again, but it might not be such a bad idea.