These are some Modern Warfare 2 Details that have been leaked so far. Some great info on the upcoming call of duty title.
-All perks will return from Modern Warfare 2 except eavesdrop, and others will be altered.
-All game modes will return from the new Call of Duty and all will have hard core versions great news for hard core fans.
-A New game mode will be in place called Secure.
-This will be a Call of duty version of capture the flag, however you capture the enemy's Intel.
-A new game mode with an unconfirmed name, similar to search and destroy, however instead of trying to plant a bomb, you try to kill a specified enemy.
-Solider customization for each class will be available.
-Sniper's with silencers will NOT be in the multi-player aspect of the game.
-A similar game version to Nazi Zombies will be available in the new game.
-There will be NO guest players online, 1 player per console.
-One of the more exciting new features is the new Kill cam save feature.
-Offline/Online Co-op mode active will be available in MW2.
-Vehicles will be in included to give a different gaming experience.
-Favorite Modern Warfare 1 maps will return.
-Helicopters, air strikes. Red dot sightings will come in different shapes for online play (circular, square, original).
-Total weapons in online mode are said to be around 35.
-New army tags will be available to add different elements in campaign (similar to halo skulls).
-Campaign-only weapons will be one of the features of the single player game.
As you can see there are some changes that needed to be made such as getting new guns and perks involved.