HTML5 is the next phase of the ever evolving web eco-system. After personally seeing and experiencing the power of HTML5 there is no wonder that so many people are already adopting those features now in response to cleaner markup.
JustinHough.com has adopted much of what HTML5 offers with a few areas that I personally have not delved into at the moment. However, for theming and general design work there is no reason not to start writing cleaner code using HTML5.
The first instance that sold me in the beginning was the doctype declaration. At first I thought to myself how much harder could they make it, but I was happily wrong in the end to find out all you need is this: <!DOCTYPE html>. Done, final, and now we can move on.
Most of what HTML useful is still there and most of what has changed are the additions that were made to the basic structure. Now code markup is more semantically based rather than using CSS classes to segment data. Tags were created such as, header, nav, article, section, aside, footer, video, etc. You can see a small example below of a basic page markup in HTML5.
I will be honest I enjoy creating new themes and experimenting with cool new features for websites. This sites new theme was an experimentation of HTML5 and CSS3. A tad there and a sprinkle there. In the end I enjoyed the new pieces of what I could do, but in looking back I know that could start separating content into easy to manage chunks that were not only easy to understand, but easy to read code wise.
For example, in the snippet above you will notice that in the main content area I have a header tag. You might think that this is only something you might use for the main header of a page. This is where the code works its magic. As you can see now I know that the header to my content is an H1, and a simple paragraph. Breaks up my content and my styling needs to know that content placed here is meant for a different purpose or to highlight a major portion of the page. There are more in depth examples that you can read about on @boblet's blog "HTML 5 structure - HTML 4 and XHTML 1 to HTML 5" if you are more interested in the finer details.
I guess I was more interested in letting readers know that you can start using it now and not be scared about it. You don't have to understand everything and anything about HTML5 to be able to capitalize on its usage. What you should know is that when you start using it start using the pieces that you need. On this site I don't use the <video> tag yet and a few other pieces of HTML5, but I can still utilize the parts that I need to make a great HTML5 website.
Try it for yourself and see where the experiment will take you.