XHTML vs HTML5
Every so often these days, an XHTML fanatic goes off on a rant about how HTML5 is all a horrible mistake and a nightmare (this one is about typical for the breed, and is what set me off in turn). These people are committing a simple mistake: they misunderstand the nature of the world.
It's really simple; XHTML and HTML5 are entirely different sorts of standards. XHTML is an invented standard and has failed because it had very high costs of implementation and use and provided almost no functional difference from other standards (both formal and de facto) in a crowded and mature field (that field being web pages). This is pretty much what you'd expect.
(Some people will protest most strongly that XHTML has not failed. To them, I note that something like 70% of the web browsers currently in use can't display XHTML by default, even if content authors sometimes get fooled about it.)
By contrast, HTML5 is mostly taking a documentation and coordination approach; this makes it much less risky and much more likely to succeed. Since it is taking these approaches, it is in no position to throw away backwards compatibility with HTML4 and do things like venture into the grand world of completely well-formed XML.
(Never mind that the grand world of completely well-formed XML is not realistic.)
Complaining about HTML5 not inventing things is missing the point (and misunderstanding the world). It's not that sort of standard, and it's not that sort of standard precisely because invention standards have failed in the browser; manifestly, you cannot get browser vendors to pay much attention to them (especially the 70% gorilla of Microsoft IE). The people behind HTML5 learned from the failure of XHTML, even if the XHTML fanatics have not, and to be blunt, the result is that HTML5 might just get implemented in a useful way.
(See also Mark Pilgrim, where you can find out how to use HTML5 today on common browsers.)