XBMC 13 will be heavy on new and improved functionality just as with the major releases of the past. XBMC 13 will improve the PVR support throughout its stack, GUI improvements, Stereoscopic 3D rendering support, support for H.264 multi-threaded video decoding on the CPU, upgraded FFmpeg stack, a unified buffering capability, library support improvements, continued work on the AudioEngine, and various other improvements.
Linux-specific work includes VA-API handling for resolutions larger than 1080p (e.g. 4K Ultra HD), improved ARM support, improved OpenGL ES + EGL support, improved VA-API support, improved VDPAU upscaling/deinterlacing, improved OpenMAX video acceleration, hot-plug support for Linux input devices (including Bluetooth devices), improved 64-bit support, and SDL is no longer a requirement for X11 platforms.
While it will still be a while before the XBMC 13 release, details on the current changes already queued up for this first major release of 2014 can be found via the XBMC.org Wiki.