As much as I loved Call of Duty 4's single player campaign and Call of Duty: World At War's multiplayer and co-op stuff, I think it's the Zombies mode from the latter title that has most stuck with me on a nightly basis. During the day I'll find myself thinking up new strategies or weapon combinations that might help me (and my friends) make it past round 20. But what is so, for lack of a better term, compelling about this game mode??
Much as most of this "fun" can be attributed to how cool zombies are as foes in most games, I think there's something else at work. Modern games like Left 4 Dead and Dead Rising have all but perfected the tension and bloodthirsty actiony fun that games like Resident Evil (and the Mercenaries mode of that series) started. Call of Duty: WaW's Zombies mode has these elements, to some extent, but this "something else" that makes it as good as those games is an inherent sense of old school arcade fun. Since, as far as we know, the Zombies mode goes on forever until all the players die, you're basically playing it entirely for fun and/or the challenge and/or high scores. Think about it.
We could trace the history of games that tried to be something more than mere entertainment, but what I'm getting at is that the earliest videogames were like the earliest movies. That is to say, intended as something new, interesting, and thrilling/entertaining. There may, arguably, be a narrative to Space Invaders or even the original NES Super Mario Bros., but these games were more about having fun, overcoming challenges, and racking up points. The Zombies mode in Call of Duty: WaW returns us to this game design philosophy. There are even 'powerups' and arcadey elements like a random weapon box that you spend points to gamble on. That this mode can also be co-operative with up to three other people lends it another old school, arcadey layer, bringing to mind classics like Gauntlet and the early 90s Konami beat-em-ups like Teenage Mutant Ninja Turtles.
It was suggested on a recent 1UP podcast that the Zombies mode could be released as a standalone downloadable game on Xbox Live or the PS3's PSN. This makes a lot of sense to me because this game mode is in keeping with the classic/retro styled games on those services. Games like Galaxy Wars and Castle Crashers in which, to reiterate, the focus is on fun, overcoming challenges, and getting a high score. It's nice to see that even big budget blockbuster titles can let their hair down and get a little silly from time to time.