Speaking across communities

XML crosses boundaries XML isn't the kind of technology that's content to find a niche and work there. XML benefits substantially from "network effects," where the value of a technology increases with the number of people using it, and XML users benefit from the same effects.
Open and closed source XML offers advantages to both closed and open source developers. Philosophically, its "open" approach may not sit well with vendors used to controlling formats, but it makes enough things easier that they've been willing to compromise. Open source developers may feel their formats are open already, but the opportunity of easy interoperability seems too big a thing to pass up.
Programming communities While a lot of the early work on XML was done in Java, there are strong C, C++, Perl, Python, Ruby, and even C# communities focused on using and integrating XML.
Developers and users XML potentially opens up data modeling to people who use data, not just programmers. We haven't seen much of a "Microsoft Access" effect yet, but I'm still hoping. Open source components might be a good way to help people move from the data modeling to implementing interfaces and applications for that data.