*This was a response to the article about linearity in FPS , but since that went a bit long I will repost it here so we can have a better discussion and see what you guys think.*
There is a good essay out there (I can't find the link right now), that explains that old FPS, specifically, Doom, Quake and the like were actually a different genre from modern FPS.
Old FPS were basically first person arena shooters (think robotron, beat hazard, geometry wars), that means that the baseline mechanics were somewhat different. The main difference is that in old FPS you can dodge the bullets, remember that in those games you could actually see the bullets and they were slow moving, that made the games more reliant on movement, which means that the levels had to be open in order to give you a chance to dodge the attacks.
With the advent of more realistic shooters, in which bullets were no longer visible, necessitated a change of the underlying design of the game. Movement became les about dodging bullets and more about getting to cover or maneuvering to blind spots, that also meant that wide open spaces became a frustrating to the players since now it gave an unfair advantage to enemies, after all you became a sitting duck with basically no way to fight back.
So maps had to change and they became more constricted with a focus on cover, and no, I'm not talking about a cover mechanic, I'm talking about places were you could stop and give you a chance to survey your environment and find enemies with relative safety. Because cover became so important, wide open spaces were had to change a lot, first, they put a lot of obstacles and stuff you could use as cover, and enemy placement became much more important and had to be carefully placed to stop them from overwhelming the player(or sometimes, limiting the number of enemies). That also meant the wide open spaces had to remain largely independent of each other to prevent the enemies from swarming the player.
But just making various open spaces separated by doors or barriers would not work, that would massively break immersion and/or become repetitive, which meant that smaller spaces had to be put to connect them and provide a sense of progression, those are "corridors". Those are important for two reasons, one it serves as a break from the action, and two it makes you feel that you are advancing to a destination. At first, they were just that, corridors, but it worked better to add some enemies and challenges to keep the immersion and the pace of the action. (Pace in this sense means that the action does not stop completely, rather than a directed experience). Also this let them add interesting diversions between the big action pieces, driving segments or more commonly, scripted sequences.
All that meant that carefully crafted "corridors" were made to connect those "open spaces" which now served as bosses or big set-piece moments. With time, the lines between them blurred to what we have now, the corridors and open spaces flow so seamlessly into each other that they feel as one, well that and the corridors evolved to feel less restricted so they typically add a a few alternate routes or make it pseudo open, thus keeping the illusion of openness until the true open space appears. Or as with call of duty, they made it so that there are no open spaces or corridors but a hybrid of the two.
Also it's interesting, the exploration aspect of old games was a clever way of making you go through all the level and kill every enemy, which was the main objective of arena shooters. But since now you were in first person, they had to make it interesting to go from one place to the next, thus the world was better designed, mars colony, hell, etc, basically places which were interesting and would feel cool to explore. With time this meant that the world became an important part of the game design, with the advent of narrative importance a new use was found for the levels, to help tell the story of the game.
This brings me to an important point, objectives. For example, in Bioshock, the main objective is for you to experience the story and world, which is why they let you explore so much, the world itself is part of the gameplay, so they designed the world with that in mind. Incidentally this is also why there are many enemies which use melee attacks or special attacks that you can see and give you chance to dodge.
COD on the other hand, has a very clear objective, experience the combat and the spectacle of battle, to them, exploration is incidental, so they design the world to discourage exploration and funnel the player to he next action piece, that is why their maps seem like corridors, their is simply nothing extra there apart from the enemies. Though sometimes they do take it too far.
It's interesting how such a small change, speed of enemy bullets, managed to change the design of a whole genre.