First off i said nearly flawless, nothing is perfect, so lets just get that out of the way.
The last three years winners were Skyrim Red Dead Redemption and Uncharted 2
Aside from a few bugs in Skyrim these games run smooth (from what I have experienced). And these are games that have huge environments with all kinds of different moving objects. Walking Dead doesn't run smoothly and there really is no reason why it shouldn't.
I never noticed any recognizable instance of The Walking Dead not running smoothly. The only "bugs" I really saw were when it would take a few seconds to load from one scene to the next so it looked like it froze, but that didn't bother me. And it had some clipping issues and stuff like that which is pretty much in every game I've played. Plus, while I didn't have any major issues with Skyrim, it was a big story that it lagged a lot on PS3 when it came out. And it's at least more buggy than Walking Dead.
Metal Gear Solid 4 : Guns of the Patriots! (4/15/06) Damn the Patriots!Sneak Out 6.12.08 - SNAAAAKE!
Walking Dead really deserved to win. It's the most emotional and affecting game I've played in all the years I've been playing video games. Plus all the competition this year was lackluster, Mass Effect was a rushed game with cheap stats to make you emotional, like meeting a child for 5 seconds than the game expects his (incredibly obvious that it was coming) death to due your throughout the game. Dishonored was great but felt like one linear level to the next, with exploration and a different way to reach the same conclusion the only non-linear path. Borderlands 2 was... Well, I hate Borderlands so I'm not getting into that. All in all, I'm just saying that when a downloadable game that costs a third of the price of a retail video game... That's a pretty damn good video game.