I don't know what standards they go by, but seeing as how COD4 got GOTY last year, I can't imagine the standards are that high
While not all of this year's games are out yet,and there might be a shocker by Christmas time, so far I would give it to Fallout 3.
The main reasons are that in all typical categories: visuals, sound, physics, etc. it's on par with, or slightly above it's competitors (in any genre), but where it accels and others utterly fall down, is the story as well as the interweaving stories or completely detached stories. There's some long drawn out instances in Fallout 3 that have nothing to do with the main Vault quest. Even if the main Vault quest didn't exist, you could happily enjoy the game just the same.
That to me puts it above the rest.
For the other contenders, that we know of so far...
Warhead : Takes Crysis and makes it look even better, hard to imagine, with the limitations on graphics development these days, yet it's longevity and story line is short-lived.
Far Cry 2 : A nice change of environment from the typical shooter games, however it's very redundant and the main story is weak.
Red Alert 3 : It's gotten too many mixed reviews and responses from both RA fans and newcomers alike. I don't think it captures the essence of the original games.
Left for Dead : A great game to jump on 'bash' with, but definatley not solid enough to capture GOTY.
Stalker Clear Sky: It's definatley an improvement over the original Stalker, and would be my next nomination for GOTY.
COD5 World at War : Another enjoyable but short lived single player campaign; though the theme of always being Allies against the 'evil' of the world is a bit passe these days, and extremely limits development of story lines. The multiplayer is even worse than COD4.
Assassin's Creed : The combat, while basic was pretty fun, and the horse riding and horse control was the best in any game ever, but the repetitiveness of the story and the gameplay, killed it.
Spore : Love it or hate it type.. it can really hook once you get it going. Having to go 'online' to get the full experience is not an acceptable mandatory for a lot of people. Innovative in some ways, but not ground breaking.
I can't think of a sports, racing, flying, puzzle, etc. game that really stands above the rest.
MMOs unfortunatley cannot qualify in my mind, because they're constantly undergoing change.
We've still yet to see Fable 2, Elveon, GTA IV, Alan Wake and a few others, however I don't see them bringing anything new or impressionable to the table.
STALKER gets mine but most major game sites don't like it because it was not hyped and they hate games with replayable single players. And reviewers don't have time for online so Left 4 Dead wont get many.