(Note: This is a pretty fragmented post, I was thinking about a bunch of stuff and cohesion suffered, so don't take it too seriously. )
Ah, Doom 4... Well yes, unfortunately we'll never have a game like Doom again because no company has the balls to make the player be able to run at racecar speeds and sacrifice graphical fidelity to be able to put hundreds of enemies on-screen at once.
Doom 4 looked pretty linear to me when compared to the original Doom with it's hubs and optional side areas. Just think about E1M2; the maze area is completely skippable. Today creating a similiarly sized area would take days of work from a smaller team of people so making it optional would be out of the question; with that much effort put into it the player has to be made to see it.
This is why I like vanilla Doom. It's the perfect example of Less Is More. It has nine weapons, about fourteen monsters and mappers were able to create incredible versatility with just this. The engine doesn't even support room-over-room. After playing a bunch of megawads, many of them will feel like completely different games, just because of the way the encounters were designed. You can have encounters ranging from 2-3 enemies at once to 200-300 enemies at once, and it works.
Imagine you are in a huge room, and imps start flooding in. You keep dodging their fireballs, when hell knights appear. They are tough so you can't dispatch them immediately, you have to keep dodging their projectiles while you are thinning the hordes of imps. You can get tricky and position yourself to cause some friendly fire and infighting to keep some of them busy at least. Suddenly wall panels drop and you are surrounded by a bunch of shotgunners and chaingunners -- they immediately jump to the top of the to do list. Gotta find some cover, look for some walls, or even hide behind the knights to kill to birds with one stone. After you cleared the room and finally got your keycard you hear the noise of arch-vile cackling as they spread out in the room. You start figthing them in a panic, trying desperately to keep out of their line of sight, as they start undoing your messy handiwork.
Everybody owes it to themselves to put on some rock music, get some tough megawad and just charge in. When you get high on adrenaline as you are dodging a shitton of enemies while half dead and you notice that the booms of the super shotgun are in sync with the music... it's a feel.
Music recommendations for shooting:
Nine Inch Nails - Wish
Wolfmother - Dimension