HTML5 is not just easier to code and understand. It's also easier for search bots to crawl. The markup has less "cruft" and fewer unnecessary elements, and is mostly free from the "div-itis" seen on many websites. Other than their classes and IDs, <div> elements don't have much semantic meaning to a page crawler. However, as far as we know right now, there are no direct search benefits of using HTML5. Google isn't going to boost your rating if you recode your HTML4 site in HTML5. But, page crawlers may be able to process things a bit faster, as most HTML5 sites tend to be cleaner and easier to parse. Google and other major search providers will need to update their search algorithms as more HTML5 sites appear on the web. Cleaner markup also allows the pages to translate across a variety of browsers and devices more easily.
One of the biggest benefits of HTML5 is that it improves page segmentation, meaning that the different yet common parts of most websites (headers, footers, navigation menus, main content areas, sidebars, etc.) are treated as separate entities in a hierarchical fashion. There's no way currently for a page crawler to read your <div> ID or class and know for sure what it will find within that element of the website. With HTML5, there is a much more straightforward, what-you-see-is-what-you-get ordering and labeling of common website parts. The search engines can now more easily and consistently judge the content on your page for relevance and importance when someone makes a search, helping the user find what they are looking for faster. Good things!
Is HTML5 absolutely the way to go when you are building a new website? There are several factors to consider. What is your target audience? If they are using older desktop browsers for the most part, chances are they will not fully/properly support some of the cool features on your HTML5 site. Also, it's a standard that is in flux, and not set in stone. Validating HTML5 at the moment is somewhat difficult, and may have to be revisited when the spec is finalized.
However, if you are building responsive or adaptive websites it is absolutely the way to go! Gone are the days (for the most part) of simply needing a desktop and mobile (read: smartphone) version of your website. Who knows what devices are coming down the pike. Your refrigerator or toaster may contain a web browser in a few years, and who knows what size its display will be. The best way to serve up one responsive/adaptive website to your audience and their many web capable devices is with HTML5. Along with enhanced interactivity and media tags in HTML5, more and more content is being brought out of Flash and able to be crawled. Use CSS3 for simple animations and media queries to adjust your page layout based upon the available device canvas. Use jQuery image galleries whenever possible. Put as much of your content onto the page in a crawlable format as possible, and use HTML5 to do it! It's a way to reach as many devices as possible while allowing for forward thinking practices and page structure.
Andy Carolla - 2012-09-14