The two major features in 6.4 from a client standpoint are AppRF 2.0 and DLNA support in AirGroup.
AppRF 2.0 adds deep packet inspection to allow you to block, throttle and prioritize applications on 7 series controllers.
DLNA AirGroup support allows standards-based L2 discovery to work across subnets (SSDP/DLNA). This will allow support things like Chromecast, Roku, standards-based printer discovery etc.