Elements and attributes are pretty much atomic, and it's hard to break their cross-implementation compatibility. However, functionality that exists as a standard may have been removed from the document definition and placed in a proprietary API, immediately limiting the portability of the document. Not everything XML is open. Throughout this year, XML libraries for various platforms have been in development. The major database vendors are adding XML capability.
Libraries exist for most programming languages.
Web browsers are adding XML awareness. Yet mainly these XML facilities exist as an add-on to existing products. The true XML revolution will happen when XML handling facilities are an integral part of language and platform distributions. When XML is that ubiquitous, the real excitement can start. Most developers are in the business of enabling productivity.