DROP IE8

HTML5

• Form validation
• Geolocation
• <video> && <audio>

CSS

• border-radius (full support : no graceful degradation/weird HTC-based solutions anymore)
• box-shadow (same thing)
• opacity (without extra syntax)
• background-position's new syntax (http://tanalin.com/en/blog/2011/09/css3-background-position/)
• @font-face : no more EOT files (WOFF may even be enough at all ?)
• rem font sizing
• Alpha channel in colors : rgba, hsla
• Advanced selectors : :last-child, :nth-child, :first-of-type...
• :empty : https://developer.mozilla.org/en-US/docs/CSS/:empty
• :not : https://developer.mozilla.org/en-US/docs/Web/CSS/:not
• calc() : https://developer.mozilla.org/en-US/docs/CSS/calc
• Multiple backgrounds : http://www.css3.info/preview/multiple-backgrounds/
• background-size : https://developer.mozilla.org/en-US/docs/Web/CSS/background-size
• :checked : http://css-tricks.com/the-checkbox-hack/
• Canvas
• SVG (THIS : http://coding.smashingmagazine.com/2014/03/05/rethinking-responsive-svg/)
• transform : https://developer.mozilla.org/en-US/docs/Web/CSS/transform
• proper syntax for ::before and ::after

JS

• jQuery can be avoided for basic stuff (and 2.0 is usable if needed) :

$ = document.querySelector.bind(document);
Element.prototype.on = Element.prototype.addEventListener;
$('.lol').on('click', function() {});

• Websockets
• Element.prototype.getElementsByClassName
• getComputedStyle
• Node.prototype.addEventListener
• Function.prototype.bind
• Object.create
• Object.keys
• Array.prototype.forEach, Array.prototype.map, Array.prototype.filter, Array.prototype.reduce …
• and nearly every ES5 features (except strict mode)