COD4 did a couple things really well in my eyes:

First, it's pacing was damn near perfect. Perfectly spaced checkpoints, a great and well-thought out mixture of high intensity scenes (prologue scene on the ship, -------, etc.), slower paced stuff (--------, ---------, sniping missions), and just plain different experiences (vehicles come to mind).

Second, absolute masterful use of the first person viewpoint. I was utterly immersed from the moment I began playing the game. This was affected by the pacing to be sure, but a lot of it was the way that your controls, actions, and feedback (in the form of blurriness of the screen, etc.) reflected the situation you were in so perfectly. For instance: that big cinematic opening credits scene where you're controlling a guy that is bound and gagged as you're being dragged through the streets, and eventually -----------. Or the moment ---------------- These were well-crafted scenes, and this quality was consistent - it didn't feel like it was isolated to the climactic events.

Third, the atmosphere and context, although not terribly complex, were well done and quite entertaining. I felt like I was playing real life soldiers in Black Hawk Down-esque situations. Obviously real life military is nothing whatsoever like what COD4 depicts, but they successfully suspended my disbelief to the point where I bought into their fiction unquestioningly. This was accomplished partially through the aforementioned pacing and first-person, but also through high-quality voice acting, sound effects, the feedback when you shot your guns or stabbed an enemy, etc. All those little details really add up, and COD4 was extremely detailed.