I (like many of us, I assume) listen to a lot of podcasts.  In those podcasts there has been lots of discussion about whether or not Left for Dead 2 is coming out too early and how awesome Call of Duty Modern Warfare 2 is going to be.  In a confluence of playing Okami and listening to the Mobcast discuss excessive multiplayer in games.  Then it all came together.  We need a new Cal of Duty, we don't need a new Left 4 Dead. 
 
Despite CoD's epic multiplayer, at its root there is  a single player campaign with a story.  Prior to hearing about leveling up techniques for CoD4 I heard about a scene where you dropped a bomb from a plane through one of those "scopes."  The scene was described as disturbing because of how impersonal it felt.  Point being, there was a story that was on some level compelling people to play it and experience emotions.  This is why the "cycle" of games exists in the first place.  You play a game until you have exhausted the story, and then it is time to come out with a new one.  Who cares if game #2 is on basically the same engine as game #1 (see GTA: 3, Vice City, San Andreas) because you were doing things with a new character, or in a new story, or in a new setting.  It wasn't about the mechanics because the story defined the experience. 
 
Let's look at Left 4 Dead, or perhaps Team Fortress 2.  There is no story, despite the fan-fic people want to write about how a BLU Pyro can befriend a RED Scout.  The "story" exists in the wonderful happenings during a game with multiple people.  That story is new each time you start the game up, and as a result the amount of time the game can last before the story gets "stale" is extended.  This is the case with Left 4 Dead.  They punch in a "story" with the cool movie idea, but it isn't a narrative, but rather a situation definition that gives structure to the story you create.  The need for a new Left 4 Dead would be based on mechanics.  "Look at what we can do now!"  Better match-making, new net-code, upgrade systems, Forge, all of these are reasons to have a new multiplayer game.  But many of them can be handled with patches to existing games.  Aside from a total overhaul, there is little reason to have a new disc in your hands.  If that was the case, you still might as well wait for a while since your old product is still kicking and creating great new stories.
 
So we need a new CoD.  The time is up on the old one and there is a story in there that will be compelling.  We don't need a new Left 4 Dead.  Their "stories" are still being created a new, and while new features/modes sound like a good idea, they can be handled with incremental improvements rather than a total overhaul.  Will I end up buying L4D2 anyway?  Probably not.  I'm poor, and I'm still having a blast with TF2.  But as soon as Episode 3 hits I'll buy it for the story.