I think the most helpful new feature of HTML5 is the clearly defined parsing and error handling. Whatever markup you're dealing with, whether valid or invalid, it is guaranteed to produce the same DOM in every browser that uses an HTML5 parser. Any script you then write for that page will be dealing with an identical tree of elements.
But really, the most helpful feature is always going to be whatever one is useful for the app you're building. HTML5 gives you a new set of tools, the tools allow you to do some things you've never been able to do before, and they allow you to do some things you've been doing for years in a much cleaner way, but it's not the tools that are important, it's what you build with them.