Nevermind HBO Go, Hulu Plus, or Pandora. The app that could show the depth of Chromecast as more than a short-range, media broadcasting dongle is TicTacToe.
Last updated on the Google Play Store in October, this iteration of tic-tac-toe requires two players, and works on both Android and iOS. Once you and a friend have installed the game on your smartphones, you'll be able to connect to the Chromecast and play X's and O's until your thumbs fall off.
The two-player nature of the game foretells a possible future for Chromecast, where the increasingly higher quality of mobile gaming apps helps Android and iOS supplant the standard gaming console. Also potentially at risk are newer microgaming consoles like the Ouya.
Assuming game app developers build in Chromecast support, it'll be hard to imagine people paying $100 for an Ouya. A $35 Chromecast could allow the phone to be the game controller as you view the game on your TV.