Based on the vidoes I've seen
So you haven't played it then.
For example, the new system where you can cripple monsters by targetting key weak points to disable their weapons. That's pretty cool. What's perhaps not so cool is it's not so much a "can" do as it is a "must", because if you don't shoot the tail off every arachnotron, the flamethrower off of every mancubus, etc, they'll wreck you in short order
Absolute nonsense. You can dodge the arachnotron's guns and wear it down from a distance, redeploy to a different part of the combat arena to focus on another threat, break line of sight, or just get really close so the gun can't track you, and kite that fucker down. Or ice blast him.
The mancubus? You can easily fight it normally from a distance. The flamethrower only comes into play when you get too close, and even then you can ice blast him, and every other enemy, so you could unload all the guns you want from point blank. There are other weapons to stagger enemies, as well as the blood punch.
You do not have to target the weak points on monster, but you can if you like precision shooting.
I prefer dashing, dodging and repositioning, so I very rarely felt the need for precision shooting like that, just redeply and unload all my guns from a different spot.
Finished the game on Ultra-Violence Extra Life Mode with over 40 lives to spare. It may be possible that Nightmare difficulty requires precision shooting like that, but for anything less than the hardest difficulty, you absolutely do not need to targed weak spots.
The same thing comes up with the ammo/armor/health system. Ammo particularly, because now that the chainsaw basically has infinite fuel, getting more ammo is just a chore now. You have effectively unlimited ammo, you just have to push a button every once in a while to maintain it. It starts feeling really, really regimented and managed, which is ironic for a game that's trying to get away from the modern shooter system of highly regulated, samey combat.
It's the exact opposite. Normally you'd find one gun you like and just use that thorough the game. Low ammo count forces the player to use different weapons in a frantic firefight. A chainsawable enemy isn't always on hand, so you may need to use a weapon you aren't that comfortable with, and get used to it quickly if you want to survive the encounter.
In Doom 2016, at later parts of it, you had enough ammo for everything in a single fight and you could just circlestrafe around the arena and never run out. Sure, you needed to get stuck in to replenish health (and ammo from time to time), but in Eternal it's much more pronounced. You need to be aware of the battlefield, figure out where the easy targets are to chainsaw, which ones to avoid (like a mancubus), and plot your route to the walking ammo-pack on the fly, and maybe freeze the problematic enemies for a few moments with your ice grenade, or pull yourself to them with your supashooty. When out of ammo you need to dash, jump, and weave around the battlefield like a butterfly to get to the ammo that will make you sting like a nuclear bee.
It gets the player into the thick of the action much better than 2016 ever did (and 2016 was already fucking amazing at doing that)
The gameplay also keeps escalating, and I'm not sure how long the franchise can maintain that. Doom Eternal's combat is even more frantic and fast paced than 2016 in general. It starts out more intense and peaks higher, and adds more and more stuff. In 2016, you had to manage just the combat, your weapons, your ammo, and maybe your grenades but people tend to not use those because you have that on your shotgun and it's way better. In Eternal, you have all that, plus more intense combat, plus having to do some precision shooting mid combat, plus your dash, plus your shoulder gun which is now key to the health/armor system, plus your blood punch. It's borderline too much, and I can't imagine they can go up from here because then it will be too much.
That's actually a valid concern.
To which I reply that at least the player is using those. I never bothered with grenades in 2016, and not much in Eternal at the start, but now I consider them essential.
Let's wait with how will ID resolve this in the hypothetical sequel when we actually get to play that.