COD does a lot of things right:
Single Player:
* 60fps on the consoles. Whether you're a framerate whore or not the gameplay is smooth and responsive.
* Precise shooting controls.
* LOTS of stuff to shoot. For a game that lives and dies by its shooting, COD games toss you into a billion battles over the course of the singleplayer campaign. Compare that to other more cinematic focused shooters where half the time you're watching cut-scenes or just moving from place to place shouting lingo and admiring the graphics.
Multiplayer:
* Accessible and rewarding multiplayer without the barriers to entry of other hardcore games: Its not a rock-paper-scissors class-based game, it doesn't require coordinated team tactics to have a great time, and you don't need to invest hundreds of hours of practice to take down an opponent. Kills happen quickly, but you don't have to wait on a long timer to get back into the action. Perks mean even the lowliest of players can get a string of kills and take the initiative in a match. Challenges and unlocks always provide extra meta objectives to go for, above and beyond just winning the match.
The game gets a lot of hate in these parts, but I still look forward to each IW installment because I know its always a good time, in campaign, spec-ops and multiplayer.