Except for players with any sense of FPS play are gonna jump into COD and be able to handle themselves. Again it's all about accessibility, there will always be ass clowns who can't play a game for the life of them, but you will also have those who will jump into the game quick and be kicking some ass early on.
As I said, it's not about really being number one or top of the score board, but the feeling a player gets since the game is so liberal on rewards. Your first kill and shits flying all around the screen, your quickly leveling up getting cool guitar riffs playing, your completing early challenges, it makes a person feel good. IW did an amazing thing with COD4's set up in making something that quickly addicts a person. Its all about accessibility, not really player skill because players in COD jump in and they feel rewarded and want to keep playing unlike other games where they get nothing and just see their lousy score and walk away.
The other big thing is that death is a minor thing in COD, you are instantly back into the action so your not stuck waiting for timer to respawn, your not waiting for round to end, your not sitting there for minutes watching better players. Even if you die your right back in and constantly playing.