Play is for the music app that's why. It is very specific on the commands. "Play disk" will bring up the music app every time but if you are on the home screen "play disk" will play the current disk in the drive.
Which is stupid. If someone says "Play Fifa 14" there is no reason it shouldn't think I am trying to listen to some music by artist "Fifa 14". It knows that is a game (because I can say "Go to Fifa 14"), and MS knows damn well the term "play" has been used for games since the invention of games.
There is no intuitive-ity in the voice commands. It feels like some garbage from the 80s. We are past that. I can say to my iPhone "give me directions to Taco Bell or call my girlfriend" and it will. For such a big feature, this is huge failure on MS' part.
Also, last night the batteries died in my controller. They lasted a weekend... Granted, I pretty much played Friday night and the entire day Saturday and Sunday.