WARNING: SPOILERS!!!
I just saw a full play through of a mission so powerful that after reaching a checkpoint you will be given the option to skip it. Infinity Ward wasn't joking, it's shocking. I personally think it belongs in the game though because it better establishes who your fighting, but it's pretty disturbing to say the least and I'm glad they give you the option to skip it. To summarize it all you play as an undercover CIA operative caught in a bad spot and you with a few terrorist have to paint an airport red. The NPCs playing civillians aren't the kind in say any of the GTA games. They show emotion, they have personalities, and the blood spilled off of them is more graphic than that of soldiers you'll frag and will completely cover the floor. HUNDREDS of civilian NPCs end up getting gunned down on this mission, the first wave of which you can see crying and begging for they're lives, the rest of them just trying run. I'll most likely play it, but I can imagine a lot of people using the skip option on it. I would post the video, but in addition to Activision jumping on it so damn quick it would be too graphic and disturbing. With this mission it was obvious the Infinity Ward was trying to put you inside a terrorist's shoes as well as show you some of the more horrifying parts of war.