Lots of topics to discuss, but the GOTY talk got me thinking also, since this really is the best year for games I can remember.
Red Dead, Heavy Rain, and Civ V are certainly up there for me. Starcraft 2 was too much of a retread for my taste, and I always try to reward innovation. Frankly, I largely burned out on RTS games by playing Age of Empires far too much.For the same reason, I'd not give it to Civ V... it's a great game, but isn't gigantically unique: really just polishing a great formula to perfection. Halo? I'm just not that into things that feel like rail shooters... burnt out on that around NOLF 2 (which was a great game that no FPS has come near for my taste since).
Heavy Rain was amazing, but a bit short. Red Dead wasn't perfect either for my taste, as I found it lagged a bit around the 3/4 point and didn't have much replay value (as Brian pointed out, and I strongly agree with him on that).
Even though it came out at the very end of 2009, I'd lean toward Solium Infernum for my GOTY, it's highly unique, has multiplayer (not perfect for that though as it doens't do a good job of it) and lots of replay value. I guess it also get disqualified as having not been release exaclty this year though.
I think, unless something surprising comes out before year end, it will be Mass Effect 2 for me for GOTY. The game felt extremely smooth, I got emotionally involved in it, it was a good length, and I did replay it and was tempted to start another replay... not bad at all, and the first Bioware game I really loved since BG II.
For sheer replay value, I think I'd choose the alpha of Dwarf Fortress. Put in nearly 100 hours so far, and barely feel like I've started ;)