COD4 completely deserves GOTY, hands down (a least on the Xbox side). Its the only game I've played all year that is absolutely feature complete - and doesn't feel like it's missing something. In fact its got everything you'd expect, and then some.
Multiplayer alone had enough steam to take the wind out of Halo 3 for a lot of players, me included. The perk system works much better than I thought it would, and the xp/ranking, challenges and party system take the cake. The party system has a few bugs that split your party up, but they'll be ironed out with the next patch.
The single-player campaign gets a lot of shit on GAF for some reason. But its a perfection of the formula started in previous COD games, but taken to the next level entirely.
From starting you off on a swaying cargo ship in the Bering Strait, and the "you-are-fucking-here" opening credits, to Death from Above, War Pig, Shock and Awe, All Ghillied Up, and the final few psychotic levels, the game is just flawlessly executed.
I wasn't even planning on buying this game in the first place, but damn its turned me into a near fanboy. Other devs should stop and take notice at all the things Infinity Ward nailed with part 4.
60FPS, smooth controls and awesome animations FTMFW.