For one thing, the syntax can be truly validated (which couldn't be done before). They've done a lot with DOCTYPE. They've totally overhauled and ripped about the list of tags. Depending on the MIME type, it can be proper XML format (making XHTML obsolete). The differences are quite a few, I suggest you check them out (http://dev.w3.org/html5/spec/). One sign of what's happening is how much bigger the HTML5 spec is compared to HTML 4...
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