Clearly you've either missed a beat and never played Doom/Quake 2 as a youngster, or you are currently a youngster so wouldn't know any better anyway!
Doom 4 is a throwback to the old school shooters where you had to fight your way through using your head rather than being spoon fed everything by the game and quick saving at every corner.
Many gamers are loving it because it genuinely is an excellent classically focused campaign shooter without compromise on what makes this type of shooter great.
It has great gameplay, great music, great action, lots of weaponry that you need to switch around to make progress and some RPG elements to perk up weapons.