The way I put it is that Doom (2016) is more balls-out heavy metal, whereas Doom Eternal is more of a "concept album".

See, it turns out that in Doom (1993), there was sort of this refined balance between the enemies and the weapons you carried, such that while all of the enemies could in principle be taken out with any weapon, there were generally one or two really effective ways to kill them. Demons (pinkies) for instance, lacked a ranged attack, so it was possible to kill even a pack of them with the chainsaw while taking minimal to zero damage. And the cacodemon was large, moved slowly, and had a high chance to stun, so a rapid-fire weapon like the chain gun or, better yet, the plasma rifle would make short work of one while affording it little opportunity to counterattack.

For Doom Eternal, the developers decided to really lean in to this idea, calling it the "Doom Dance", and craft the enemies in such a way that they were specifically vulnerable to specific attacks from specific weapons. Again, using the example of the cacodemon, it's a real bullet sponge but if you pop a grenade into its mouth, it's an insta-stun letting you do a glory kill. The Mancubus and Arachnotron have weapons that can be disabled or weakened with specific attacks. And, annoyingly, there was one enemy (the Marauder) that can only be killed via a sort of quick-time event.

This expands to resource management too. There are fewer pickups, which means you have to top up on health with glory kills, ammo with the chainsaw, and armor with the Flame Belch as you clear an area of enemies. The emphasis is on "using the right attack at the right time", which is what the developers were deliberately aiming for. The campaign was also much more story-driven which only adds to the concept-album feel, as it's a very eurocomic-ish story that delves into the connection between the demons and the angelic aliens known as Maykrs, rather than just thrusting you into hell and telling you to murder every demon in sight. They definitely wanted you to get the most out of the game by experiencing it a certain way.

For these reasons I liked it less than I liked Doom (2016). I can see what they were going for, but it's just not my thing. For Doom: The Dark Ages they appear to be changing the combat system yet again, with more emphasis on tanking, and dealing out, massive amounts of damage from/to hordes of enemies, as well as use of a throwable shield and a more flexible glory kill system. I think they realized that they kind of veered from the Doominess of the combat with Eternal and are attempting to course-correct. Props to them for trying something different.