Call of Duty: Modern Warfare came out nearly twelve years ago. In a medium where just six months can make a game ancient history, that is a long time. And although there have been as many entries in the series as there are years between that release and the upcoming launch of Modern Warfare, there aren't as many games as game-changing as that one was.
Among the many ways that the game was so innovative was its approach to mission structure. Whereas previous games focused on large-scale recreations of WWII battles, this entry honed in on tactical operations, involving a more intimate sort of violence. This is most pronounced in the mission "All Ghillied Up"
The mission has players switch protagonists, controlling Captain Price in a flashback which sees him fail to assassinate Imhran Zakhaev, the antagonist of the first game. A first of its kind, it was unique to the franchise (and to first-person-shooters) because it required players to think their way through the mission. Sure, it's not dynamic gameplay--it plays the same way every time--but it is tense in its pacing and execution.
In creating this level, developers had to grapple with many challenges. For one, there was lines of code. The usual amount in that game's levels were about 5,000 with bigger levels reaching up to 15,000. Scripting and enemy movement also proved a challenge (the devs had to create whole new animations just so enemies could walk).
This and more is discussed in an interview with the Infinity Ward developers responsible for the level's creation in a video by Game Brain.
Running the gamut of development from their first and second entry all the way up to 2007, the interview spans developers' inspirations and setbacks. One of the reasons why the player just teleports into the bird's nest near the end of the level (where they shoot Zakhaev) is because the developer just ran out of time!
All that and more are on the plate in the interview.