The Future of HTML

IBM's developerWorks has an interesting article on the future of HTML, mentioning HTML 5, a proposed progression to the HTML standard that is backwards-compatible with HTML 4.01 and XHTML 1.0 (I can sense you nodding off already). They also touch on Web Forms 2.0, which would extend current forms implementations to include built-in client-side validation (so you don't have to write custom JavaScript for required fields, or validating an e-mail address). There seems to be a lot of interest in what the web is transforming into, after Google showcased what you can do with AJAX to create a more compelling user experience. So what's the best approach to a more engaging and useful web - AJAX, Flash, the next generation of HTML or XHTML, or a combination of all these things?