Like you said, this is really early to be asking, but so far, my favorite game of 2011 is undoubtedly Mortal Kombat. I honestly can not remember the last time I played something this violent... or fun.
It's also a whole lot more forgiving to me than other fighting games, including the original Mortal Kombat games. It's not so easy that it isn't a challenge, but easy enough so you're not bashing your head against your controller. One thing I've noticed is that it's a bit more balanced out than previous Mortal Kombat games. Sure, some characters are really overpowered, but as far as I see, the ones that are that way are meant to be.
I could go on about how fun it is, or how brutally amusing the fatalities and X ray moves are, but I'm going to point out one thing I really noticed: The character personalities.
I could never tell what the character's motives or personalities were in any of the MK games, save for the assumption that Scorpion was really angry all the time and that Johnny Cage was a douchebag, stuff like that. In this new Mortal Kombat, everyone seems like their own character with their own individual personalities which can be seen easily through their fighting stances and moves alone: Scorpion is the intense hate filled badass, Johnny Cage is the cocky showoff, Noob Saibot is the cold, heartless spectre, and Mileena looks like she's childishly enjoying everything too much.
Due to this, I've suddenly gained a few new favorite characters now that I can tell what they're like. I've never really cared for Kabal, but now that I can actually understand what his problem is, I like him a lot more.
Another reason this seems like it will be my game of the year is because there's not a lot of competition. I haven't gotten Portal 2 yet (shame on me), and Duke Nukem Forever comes out next month, but other than those two, there's not much I know of to compete with Mortal Kombat. There is that new Silent Hill coming out I guess, but it doesn't look very promising as of now. Maybe if they did what Mortal Kombat did (go back to it's roots and attempt to recreate what made the classics, like Silent Hill 2, so great), Silent Hill would rise again to it's former disturbing, rusty, blood stained glory.