You know, when I hear Yahtzee talk about these linear FPS games that offer no freedom of choice whatsoever, I'm reminded of another kind of game: rail shooters. Those too are often about moving from one predetermined point to the next, shooting at the enemies as they appear, then moving on. The thing is, I've liked rail shooters in the past; you don't see many of them these days (as a result of the sad trend of eliminating video arcades from malls, thus depriving gamers of one of their few excuses to get out of the house and socialize with other gamers) but back when they were around, I enjoyed them, the House of The Dead series in particular. But the model used for rail shooters does NOT work with FPS's for two reasons: speed and control. In the words of an on-air personality on my favorite classic rock station, let me preach on it.
In a rail shooter, you are moving down a pre-determined path (sometimes with the occasional alternate route to be discovered by shooting a switch to lower a bridge or hitting an elevator button or something along those lines) at the game's pace. Being guided in this way means that you're on an active, adrenaline-fueled roller-coaster ride and you have to keep you eyes open and ready 'cause you never know when the next terrorist or alien or zombie or personal injury lawyer is going to pop up and rush at you begging for a bullet to the face. Rail shooter's realized that if you're going to hem the player into a predetermined path, you need to make sure the surprises keep coming and the player doesn't have time to get bored.
But when you put this style of gameplay into an FPS, you don't often get that same feeling of exciting tension. Instead, since the player is in control of their movement speed through the game, they can inch forward, get 1-2 enemies to spawn, then fall back so none of the rest do, shoot the few enemies that show up, then push on. The only way this gets broken up is if the game has ambush scenarios, and usually you can see these things coming a mile away from the layout of the terrain, walking into a big canyon-like area with scattered walls for cover, unless the entire level is designed like that.
Plus, rail shooters always give you a chance to hit the enemy that's about to attack you, if you're quick enough on the trigger and your aim is good. With a FPS, you could be concentrating on one enemy popping in and out of cover in front of you while his friend plinks at you from the side. That, to me, doesn't seem exciting, it's just annoying, like the kid you sat in front of in class that shot spit balls at the back of your head while you were busy trying to do the same to the one in front of you.
That brings me to the other problem: control. Now I don't mean controlling your character's movement this time, I mean the means by which you fight back. In modern FPS's, this is either the analog sticks on your controller or your mouse, the latter of which I find infinitely superior, as FPS's go. But even mouse shooting isn't as fast as the good old-fashioned light gun, weapon of choice for arcade gunslingers everywhere. The modern FPS isn't designed to be played with a light gun, and for good reason; how would you control movement? I suppose it's possible someone could develop something like a foot pedal that would stop and start movement and design a light gun so you'd turn in the direction the gun was pointed (Note to self, write suggestion to Microsoft, Sony and Nintendo).
The problem is without the light gun, enemies can't be made too fast or the player won't have a chance of hitting them. It takes longer to move a mouse-controlled crosshair to line up on a target than it does to swing a light gun to the side and pull the trigger; not much perhaps but just enough that your average rail shooter enemy, with their typically-erratic lurching or their tendency to lunge at you from every ridiculous angle possible, would quickly tear apart someone trying to shoot them with a mouse.
Now, there's two ways to deal with this problem: either slow the enemies down so like a real life person, it takes them a moment to line up their sights on a moving target, or give players options besides trudging down a predetermined path that leads them straight to One Way Bullet Hail Alley. The former route is the better of the two if you must have an extremely restrictive linear playing area, unfortunately game developers often go too far, so much so that enemies are so incredibly dense they rarely make use of cover. This is why games like the original Return to Castle Wolfenstein (2001) are much better than crap like Clive Barker's Jericho. They're linear, but within the confines of a game like RtCW, there's still some room for creativity, whereas in games like Jericho, it's actually less entertaining to play than Nintendo's Duck Hunt.