New semantic elements were introduced to make code easier to divide.
Tags like 'aside', 'article', 'header', and 'nav'
can tell a lot more about a code division than 'div' or 'section'.
Scalable vector graphicscould be used natively to give more funtionality for artistic design. From simple shapes to more complex drawings.
With the canvas element, you can draw in real time with javascript.
With the ubiquitousness of mobile computers and phones, more and more people are using the web from smaller device screens. HTML5 makes this relatively easy with responsive elements, media queries, the viewport, and the ability to easily scale elements with % and ems.
HTML 5 introduces the use of video tags without having to use annoying and insecure plugins like flash.