Conclusions

Enormous capabilities The potential of markup for expressing a wide variety of information seems pretty clear. Even where markup doesn't make sense (describing individual frames of video) as a carrier of information, it can be useful to connect different kinds of information.
A common set of tools XML parsers, XSLT, and a variety of frameworks for working with XML are available to developers using any of the markup languages described in this talk. It's entirely possible to create a set of documents in a custom markup language and distribute the information they contain using any of these formats.
Still searching for a home The client side for much of this remains depressing, and is probably the single largest barrier to XML's usefulness as a carrier for human-oriented information.
Need some leaders XHTML didn't really inspire people. SVG has a thriving and active community, and XSL-FO appears to be developing one. Getting markup beyond the browser block is a tough challenge.