Modern Warfare series opened a new window to FPS action games.
After the disappointing World at War, Activision gave the players what they want. Real players want a real atmosphere at all games no matter if it is RPG, FPS, ACTION,.. I mean not just the graphics also the story, AI, interactions, camera angles,... has to be realistic. So the game directors has to be as good as cinema directors.
Formerly, when you play a FPS, you just take a gun and kill everything you meet. Only RPGs had some character interactions and sensations. Now everything changed. All games must have these realistic parts. I don't know if this is a good thing but now you become a part of the game and live inside the game with the main character. This is actually what the players want I think.
I haven't been playing FPS for a long time because of this old FPS style. But thanks to the producers they produce games worth to be played. Modern Warfare is one of the top ones.
Note : I haven't played the multi player mode and I think it should be worse than single player. Because it is really hard to adjust multi player balance for the games. Try "Savage" if you want to play a balanced multi player game.