The thing that should make librarians happy is increased relevance of the code to the content. A header is marked <header> instead of <div id=“whatever”>, and so on. Not only is it more difficult for humans to read though and change when needed, computers have a harder time which complicates format changes. I have spent untold hours cleaning up old and unreadable code for simple website redesigns. Imagine the problems turning web content into handouts.
Another plus no one has spoken of is the fact that unlike earlier HTML tags, the new HTML5 tags don’t have predefined presentation in the browsers. When I redesigned this site I used HTML5 and it was the easiest browser verification I’ve ever had. It looks pretty much the same in all the major browsers and the problems in IE6 (which are most likely from the advanced CSS selectors I used instead of the html) don’t effect the readability of the content in the least.
If I’ve piqued your curiosity, the collection of HTML5 resources and tutorials at Speckyboy Design Magazine should give you a good start on learning the HTML specification.