To be honest, there were some really great games this year, but the only real competition came from Starcraft 2. It may be shocking that I didn't pick Starcraft when I've wanted it for so long, and am so loyal to Blizzard, but Mass Effect 2 really is that good.
I think what really sealed it for me was that ME2 actually made me cry.
Yea. It did.
It wasn't tears of sadness though. It was simply because I got so swept up in the characters, and watching them play out the whole end sequence, it was just so damn heroic. It got to me, in a totally awesome and satisfying way.
I breaks my heart when people play this game with a mind set like "all paragon/renegade" and even more so when they get every upgrade so that the whole team survives. THAT'S BORING GUYS! I THINK YOU MISSED THE POINT!
Anyway, it boils down to my own experience with the game. I've never been so absorbed in the actual roleplay of a story before, and for that, it easily gets my pick for Game of the Year.