I get what Jeff is saying about mental fatigue and being "always on 11". The same thing happened to me during Black Ops' single-player. After an hour or so of continuous play I feel my brain just turn off abruptly and realize I'm just going through the motions while wondering if I'm just bored by that those type of games now, instead of being immersed in the action. Basically at that point I need a break from the non-stop action, which Black Ops does not provide, so I have to stop playing the game for a while and come back to it later or the next day.
In that regards I think that's why Infinity Ward had those slower paced sneaking levels, like the one where you follow Price to get into position to snipe, followed by high intensity levels, like the escape directly following that sniping moment. It's not just traversal levels as Brian put it (i.e. from his tone of voice "boring"), as Infinity Ward proved they can also use those moments to establish character relationships (who didn't like Price after that flashback) and mood.
It's just realizing that people can't be at 11 all the time and giving the game better overall pacing so that players can go through the entire single-player without having to take breaks off the game feeling bored.