Just ask Mrs. Jones - and she'll tell you; I'm a hopeless CoD4 addict. Almost every night I plug in.
A first person shooter, CoD4 is set in the modern period, with today's weapons. Now for people that haven't played much online shooters, its worth explaining that this isn't like playing Castle Wolfenstein. Matching wits against other players online, with multiple game modes, is very dynamic.
As I've gotten better, so has my appreciation of the layers of gameplay. First there is understand the dynamics of certain maps. The size, traffic flow and choke points all affect how you play. Weapon and "perk" selection is equally important. Certain weapons suit close range, medium, and sniping, as do "perks" or a set of 3 extras. It might be the ability to not show up on radar, or extra ammunition. Every perk and weapon has a strength, as well as a weakness, so correctly constructing up to 5 custom classes is almost as important as gameplay. Each game mode requires a different style of play.
What really makes the game interesting, as a result of its popularity, especially among skilled gamers, is the psychology. Noting behavior and predicting it - both for your teammates as well as your opponents.
The longer I play it, the more nuanced it's become. The portrayal of onscreen violence is largely lost on me; I abstract it similar to playing Risk abstracts large-scale land wars and casualties incurred. For me - its like chess and go, mixed with tag.
The biggest drawback is playing with people (everyone has headsets) how have less than enlightened views towards the game, and towards respect for other gamers. Nothing's perfect, and that's the price of online interaction - unfettered freedom of expression with any external requirements of conduct certainly allow people to show their true colors. Of course, that's not unique to CoD4, and by and large its a respectful community.