I'll say something about the new input types and the "we still have to validate on the server side"... you ALWAYS had to validate on the server side, this isn't extra work of any sort. The possible benefit of the new inputs is, it may help steer users into inputting the correct info or type of info in the FIRST time, rather than them submitting and getting an error message. I can get behind the idea that inputs expecting a very limited type of data can be semantic and therefore more specific tags in the markup can make sense.
One reason I would NOT use HTML5 (and I don't mean the doctype on top of HTML4) is the way screen readers are fumbling with the new elements. Sometimes it's the fault of the reader. Sometimes it's the fault of the browser. None of the browsers have implemented their accessibility to deal with HTML5, even the browsers who are claiming to "support HTML5". There is a whole layer of additional information browsers send to any (possible) accessibility software that may be interacting with the browser or the computer's windowing system and that needs to be built for all these new elements and attributes. Understandably, AT vendors are being slow and cautious since, as Felgall pointed out, this stuff is still in DRAFT. Things that are very unlikely to change such as <header> tags are getting addressed by browsers and AT but stuff like the "required" attribute in form controls or how to deal with the <canvas> tag's API are still not done and won't be any time soon.
Here's just an example of the kind of stuff going on out there. Yay, JAWS 12 works! Oh noes, you have to sell your grandson to afford the upgrade from JAWS 10! That kind of thing.
If you are building a site that must pass some level of WCAG or section 508 or that new ammendment they want to add to the ADA or a commercial site in any of the countries that have started making demands on that sort of thing, you will probably want to stick to HTML4/XHTML until things settle down.