I was referring to both.
..... and by gameplay , I mean game-play systems/mechanics not game modes. Skill are developed by practice and map understanding improves over time. But the system that you use to implement your plans, the gameplay mechanic, the shooting, is very simple and easy. You can customise your gameplay using perks/abilities and custom gear in MP and play your own way, even though the basic option of actions perform-able in the game, though robust, are very limited and basic, leading to huge appeal of the game.
New comers love the simplicity and accesibility and old timers get hooked to the rewards and customisability. Learning combinations and trying them out. The basic shooting aspect is fun and well done, which is what catches the fancy initially, including the 60 fps silk smooth action, and the appeal is retained by the rewards that the game showers on you for playing ! Its an excellent formula to keep the player interested in your game. Why do you think it takes so few bullets to kill in COD ? so that every one can have fun, no one feels he is a noob ! "I can kill too ! " and then he is showered with rewards for doing it again and again ! and then he is allowed to make little changes to his stats and play
just a bit different from the other guys out there. Making him feel like a King in his own right !
He feels he has made his own game inside the game.
Its Infinity Ward's genius game design !