Another interesting mechanic in Doom was the ability to either mow down monsters yourself, or save a bit of ammo by enticing them to kill each other and mopping up the stragglers. This was widely copied in "Doom clones" of the day; the instruction manual for Marathon even gave tips on "berserking".
CoD and its cousins in the "bloody screen, so real!" genre are quite different. They're fine for what they are, but some of us who have been playing FPSes for a while feel there's something missing. There's a series of parody videos on YouTube called "Call of Dooty" which are basically Doom with CoD tropes: QTEs, tutorial levels, waypoints which walk you through the path you're supposed to follow (and penalize you for straying off the path), NPC dialogue that refers to the demons as "Russians", etc.
Both Doom, Quake and the Modern Warfare games are essentially built on linear scripted sequences like that. I'd urge anyone to play "All Ghillied Up" (which may be one of the best levels of all modern shooters) and try to tell me they didn't feel at least some excitement.
By the way, if you're looking for something that carries the Doom/Quake torch of mowing down lots of enemies at crazy speeds, check out the Serious Sam series.
A lot of modern games are still fun, TF2 still captures a good bit of what the old times were like, but many modern multiplayer games have a lot of MMO and leveling mechanics added to them which introduce a fair amount of "grinding" into the game which makes it a bit too much like work and not enough like just raw fun.
I agree that levels are more linear, but at expense of other gameplay elements. Also CoD franchise is getting sucked dry. If that happened to some other game it would get criticized more as well.
Take a look at Half-Life 1 & 2 one the best games of all time. They are pretty linear[0]. And they also have keys and doors, which have the form of scientists, switches and valves, but they are there.
[0]:http://i.imgur.com/sg1TN.jpg ( large image )
Actually, it was worse than that; I had no agency. For example, I recall spotting an interesting nook in one level, investigating where it went, then doubling back to see where the main path went. But then I found that the main path was blocked, and in fact the only way forward was the interesting nook. It wasn't the first time this happened; but the cumulative effect was that it completely broke the game for me, because it removed any incentive I had to pay attention to my surroundings - I realized I would be beaten into submission by the direction of the game, with heavy-handed clues bludgeoning me to go the approved path.
I saw the levels like evented puppet boxes. Early on in HL2, there's a bit where you pop out of a sewer, and around a corner to the right, there's a bunch of guys on a ledge shooting at you. It is impossible to kill them without going all the way around the corner and into another tunnel to "cleverly" pop up behind them. After a couple of hours, you could almost see the bounding boxes of the script triggers, it was so predictable. The forced nature of the encouters were quite different from the action bubbles of HL1.
Deus Ex had moved the art so far forward, while HL2 was stuck in the past.
HL1 was what I might call a tactical bubble game. There's one action scene followed by another, with the paths in between fairly linear. But the action bubbles themselves were reasonably open to different tactics, and especially when fighting the black ops enemies, fun. But it's a very dated by today's standards, because Freeman never talks, and indeed, has no agency himself.
So I feel justified in calling CoD a shooter on rails, because when I talk about agency, I mean a lot more than what those games give you. You do literally have control over movement, but it's an illusion; you have no choice but to go where the directors force you to go, and to the extent that you break out of your box, you're finding bugs in the experience.
Otherwise, the original Operation Flashpoint/ARMA are pretty much the ultimate FPS sandbox. As long as you are ready to eat bugs and suffer the terrible physics. Completely different from Deus Ex, obviously, but definitely not on rails.