Multiplayer, better grafix, multiplayer, less emphasis on story, multiplayer, stop relying on lore so much and let the series evolve with the times, mulitplayer, more guns and armor, multiplayer, less time designing quests and more use of Radiant quests, multiplayer, more and more waves of enemies to kill, multiplayer, preset classes cuz speccing characters is too hard, etc.
Oh, and of course, multiplayer mode. Don't take my word for it - just look at the threads.
Ok.. so let's break this down...
Call of Duty invented multiplayer, apparently.
They also invented top notch graphics, apparently.
Call of Duty invented multiplayer, apparently.
Call of Duty invented having less emphasis on story, apparently.
Call of Duty invented multiplayer, apparently.
Call of Duty invented relying less on lore, apparently.
Call of Duty invented multiplayer, apparently.
Call of Duty invented guns and armor, apparently.
Call of Duty invented multiplayer, apparently.
Call of Duty invented radiant quests, apparently.
Call of Duty invented multiplayer, apparently.
Call of Duty invented giant waves of enemies, apparently.
Call of Duty invented multiplayer, apparently.
Call of Duty invented preset classes, apparently.
Call of Duty invented multiplayer, apparently.
I'm sorry to be blunt, but do you do nothing but sit down all day? Because you don't seem to have any legs to stand on.