HTML5 & Web GL
IE11 on Windows 8.1 fully supports HTML5 video, including HTML5 premium video extensions. This allows direct playback of video in the browser (such as Netflix) without using plugins like Silverlight. The browser also supports Web GL (finally!), GPU-accelerated 3D web graphics, as well as Google's SPDY/3 networking protocol that aims to speed up webpage load time. But WebRTC (which enables in-browser voice calling/video chat without plugins) is nowhere to be found.
Microsoft has recently confirmed that IE11 will support the Google-backed SPDY protocol.