However, while the single player campaign may not be as lengthy as it could have been, that's where Call of Duty 4's multiplayer portion comes in to fill up most of the single player's slack it terms of replay time. Infinity Ward has loaded up the game with 16 multiplayer levels and support for the PC version of up to 32 players (sorry, Xbox 360 and PS3 users; you only get 18 player support). While the usual gameplay modes are in Call of Duty 4's multiplayer (deathmatch, capturing control points and the like) you also get a mode that turns the multiplayer into a more "realistic" experience; i.e. you can go down with one or two bullets as opposed to several.
That's not what makes Call of Duty 4's multiplayer interesting, however. Infinity Ward have put in some RPG-lite features that can make players go massively obsessive. In playing a match, you can get more advantages if you kill a string of enemies before dying, from better radar coverage to air strikes to at its highest point calling in an AI- controlled helicopter that fires on your foes.
Even better is unlocking special ammo, weapons and extra physical abilities the more you play the game online. The more kills you get and the more wins you and/or your team get in a match create more points for you which you can use to upgrade your rank and get more "stuff".
The game's class system starts you out with the minimal stuff but as you get more points you unlock more classes and the ability to create your own class with your own set of weapons, items and "perks". It's the perks that make Call of Duty 4 multiplayer more of an RPG experience than most shooters. From being able to take more damage to running faster to being able to pull out a pistol when killed in order to take out the enemy that killed you. It's new features like this that make playing online more fun.