As is tradition with me, I started with the single-player campaign. I wasn't expecting a whole lot, considering how I felt about the previous iterations and their own campaigns, but the first few missions played well. The plot certainly did little to keep me playing, but the first few kills were satisfying enough, if not sloppy on my part.
I love character stories, and I didn't realize CoD4's characters held any meaning to me until the death scene of the US Marine after the nuclear explosion and subsequent helicopter crash; I was stunned at the scene (the tiny bit of control they give while the character passes was powerful) and even more so at my reactions to it. I was more attached to Soap and his crew, so I pressed on earnestly.
I got a bit frustrated with the difficulty balance employed by Infinity Ward in the later levels: in a game where one or two bullets can end the player, do not force the player into a run-and-gun scenario. It wasn't challenging or fun. It was frustrating.
The last scene in the game really proved to me how subtly IW had connected me with the characters. After I looked at the clock and realized I had played the entire campaign in one sitting, I began agreeing with the numerous "Game of the Year" awards it won.