Doom Eternal is an excellent game mashed up in a blender with a terrible game. I am extremely suspicious of anyone who recognizes one half without acknowledging the other, on both sides.
It's got a lot of unwholesome aspects (4-5 layers of progression bloat, cop-out text tutorials, erratic content structuring), however the core gameplay is lightyears better than Doom 2016. My main complaint about D16 was that it didn't lean hard enough into the arena shooter style, so a lot of its new ideas (esp. glory kills) felt like band-aids to encourage active play that would be unnecessary from a classic FPS viewpoint. Eternal succeeds by doubling down. D16 introduced glory kills to replenish health and chainsaw to refill ammo, but the former was basically a free action due to the large buffer zone on enemy health that made it hard to overkill, so you were basically invincible with the rubber banding on health drops. Doom Eternal smartly makes the health window for glory killing quite a bit narrower, so you have to control your damage output to reliably heal from fodder enemies. I rarely got execution prompts on pinkies, and I even frequently lost glory kills on arachnotrons. Much ado has been made about the tighter ammo economy, but I think it's mostly a wash thanks to the chainsaw regenerating on a cooldown, which I view as a major blunder. There's plenty of ammo to be found in arenas, and I'd have found it more interesting to scavenge in real time for ammo and chainsaw fuel while retaining the choice to generate more ammo vs killing heavies as in D16. Armor is still just Health 2 The Sequel to Health as it absorbs all damage in place of health, but in Eternal it plays a more active role in that the player can generate it with the flamethrower as a temporary buffer to volatile health, like picking up a mega health in Doom/Quake.
A key difference that brings things together is that the enemies are much more aggressive in Eternal, putting much more pressure on the player to use effective weapon combos and stay on the move, making health, ammo, and armor management much more engaging. About half of D16's enemies were well-suited to arena combat (imps, pinkies, shotgun shield guys, hell knights), while the other half (cacodemons, revenants, mancubi, etc.) weren't updated to counter the player's mobility and thus filled the role of harmless damage sponge. Now, arachnotrons, revenants, and mancubi are all super threatening up close, and even imps, hell knights, and pinkies made more an effort to catch up to my dashing to threaten me -- combat overall is much more frenetic. Some enemy weaknesses are brilliant. Shooting cannons off of arachnotrons, revenants, and mancubi is rewarding and presents a tradeoff vs dealing direct damage that depends on your equipped weapon/mod, ammo count, level geometry, other threats. Other weaknesses feel ill-considered, though. Cacodemons gulp down grenades for effortless glory kills; why not require the player to time with their charging bite attack? Shield guys blow up from plasma, a dominant one-note solution; why not require the player to shoot 2 to create an arc between them?
The content besides the arena combat is hit or miss. I like the platforming puzzles somewhat, moreso than in D16, but agree with everyone who despises the purple goo and tentacles. There's a decent integration of combat and exploration with various traps and enemy spawns, but the game is still mostly arena-corridor-arena. Worse than any of that, though, is how the game makes almost no attempt to organically teach the player. I mean, it kinda does; the first map is a barebones slog with only a few weapons/enemies to ease you in. But the whole game is filled to the brim with text pop-ups, like they spent their entire dev time creating enemies and player mechanics and then gave up on introducing them through gameplay. The worst of it is when a text pop-up appears for an enemy you've never seen, completely ruining all the surprise while also spelling out exactly how to beat them... in a way that discards all the tactical depth and nuance that's actually available to you during combat. Sometimes the tutorials are even misleading -- everyone who thought you had to stay in a Goldilocks zone to combat the Marauder probably got screwed by the game implying as much. But if you don't have the text pop-ups, some of the inferences you have to make are pretty unreasonable; how is anyone supposed to figure out that the Cyber-Mancubi shields are resistant to everything but a blood punch, or that the Doom Hunter's shield can only be disrupted by the plasma gun?
The audio design is abysmal as it was in D16 -- the mix is so dominated by low frequencies (except the aggravating monster screeching), from gun sounds that sound like they're underwater to the endless djent guitar chugging, and there's no directional audio to tell anything apart. The sound cues that play when you're low on health or have a blood punch available were made ear-splittingly annoying to cut through all of the noise. The game is also stuffed with an unreasonable number of progression systems, all of which are utter garbage save for the weapon alt-fire mods. Several of the rune perks feel like things that should have been enabled by default (e.g. air control and faster glory kills), so you're left with at most 1 of 3 slots to determine a "playstyle". The wheel of suit upgrade is just embarrassing, with 3 of the 5 sections offering no-brainer upgrades like faster cooldowns and better grenades, while the other 2 sections offer useless crap like larger automap radius and respawning explosive barrels. You also get upgrades to your health/armor/ammo/capacity, which is for some reason tied to yet another layer of shitty perks that are impossible to remember or care about. The weapon mods are cool but you still have to waste a bunch of time upgrading them by performing tedious kill challenges, which you can graciously skip by using another progression item. You'll spend a bunch of time exploring for these upgrades but half the time you'll just find dumb cosmetics, collectibles and lore dumps. Story is utmost cringe but can be largely ignored and the UI is pretty unpleasant, but at least there's a red-green colorblind mode.
At the end of the day you have a pretty deep and arguably innovative combat system which is dragged down by pretty much everything else about the game, from lazy player communication and progression busywork to terrible sound design and DLC bloat. I think the combat makes it still worth playing, but whoever the higher-ups in management were who greenlit the other shit should be excised from making games.
EDIT: I forgot to mention how 2/3 of the way through, the game gives you a sword that can oneshot every enemy in the game, as if it needed another oneshot tool on top of the chainsaw and BFG 9000, with ammo for each thrown at you ever more regularly. So even as the fights get more complex, it shoots itself in the foot by allowing you to just delete elements of the challenge.
