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What's so different about HTML5?

There's a couple of great new approaches that are taking place in the HTML5 description.

The Rise of WebApps

First, the language is being built around WebApps...small focused applications that can run on a browser or as a mobile application. The new version of HTML has features like offline storage or the ability to handle data even when the app is no longer connected to the internet, geo-location or the ability to detect and work with the location of the user as well as excellent rich media support...providing easy to implement audio and video elements.

More descriptive semantics

The original goal of HTML was to develop a language that could describe the structure of a document. As the web grew, developers realized that oftentimes they were describing the same types of elements. For example...a header, a footer, nav, articles, sections, etc. HTML5 recognizes where the language has developed and incorporates those elements into the language with new tags for that type of content.

Richer Media Elements

One of the great new improvments in HTML5 is the development of rich media tags like audio and video. Fully programmable with JavaScript, these tags offer incredible ease of use while also offering powerful flexibility through extensive access to media elements through programming

Why not HTML5?

I wish we could all be using HTML5 today, but the reality is that there's a few problems that prevent the language from use in modern websites.

Browser Support

The main problem with HTML5's acceptance is that only modern browsers support it. By modern, I mean almost everything except for Internet Explorer. The new version...IE9 offers excellent support, but as of this writing it's not quite out of beta. Even if it were, the majority of people will still use older versions of IE for quite some time. There are things you can do to make the language play nice with older browsers, but none of them are perfect.

The Language is a Spec

Another problem is that although parts of the language are very stable, the language itself is considered a work in progress, so technically, any of the elements could change at any time. The language is not expected to be completed for several years, which complicates things further. Thankfully, a lot of the language is considered stable and ready to use. I think it's such a great move forward, that you should develop a Graceful Degradation approach to writing your HTML. That simply means writing HTML that will work with older browsers, but will offer users with more modern browsers an enhanced experience.

Media Licensing Issues

Another ugly fact about HTML5 is that because of licensing issues, rich media has to be compressed in multiple formats in order to be compatible with most browsers. So you'll probably use something like mp3 audio for webkit browsers (safari, chrome), and ogg for mozilla (firefox) browsers. It involves a bit more work and it is a pain, but hopefully those issues will be resolved soon.