The best downloadable game goes to a game that you can only download via a service such as Xbox LIVE Arcade or Steam. Usually, this is download only with no disc based version available. This category is slowly getting phased out because almost every PC game, and most console and handheld games, are becoming download only. This category usually only applies to downloadable console games, but PC ones get thrown in every so often.
The Walking Dead mainly gets the award because it is not only gripping and thrilling, but I anticipated every single episode throughout the entire year. Telltale really made people watch their calendar because they wanted the next piece of the story to this amazing adventure game. If you can make players chomp at the bit for an entire year for each episode, you were successful. The other games were great, but nothing captures the atmosphere and heart-wrenching tale of TWD.