Does anyone else take advantage of this feature? If your HDTV (and surround receiver) are all connected by HDMI to each other and aren't more than a couple of years old, you should be.
HDMI-CEC allows devices connected by HDMI to control each other. For example, my TV's remote can now control playback and volume functions on my Blu-ray player without hassle -- it all just worked as soon as I enabled the feature on both devices.
My Chromecast is plugged directly into my TV, which also supports ARC (Audio Return Channel) so the audio gets passed straight through to my Blu-ray player without any extra wires. After enabling CEC on my TV and surround system, I can start casting from my phone and the TV and Blu-ray player / receiver automatically power on along with switching to the appropriate outputs. True one-step casting.