Today W3C released the first draft for the next generation HTML language. There are plenty of differences including lots of new semantic tags (header, footer etc) , couple of APIs (2D drawing, drag and drop, network), new events and other cool stuff. Of course the guys have done great work with revising the language structure (cleaning up some tags and attributes) and still achieving backwards compatibility. Also I liked the idea of making HTML5 a XML language.
for more info check the official pages:
http://www.w3.org/TR/2008/WD-html5-20080122/ Working Draft
http://www.w3.org/TR/2008/WD-html5-diff-20080122/ – Differences between HTML4 and HTML5