Hey ya'll.
I don't usually post but felt I needed to throw my 2 cents in here. (it'll be long post, tl : dr below)
I played both Doom 2016 and Eternal, so will reference them as such.
2016 was amazing comeback for Doom (and id Software). It brought back a beloved franchise with awesome graphics and new gameplay. The fact they restarted the development was a great call (it was supposed to be a crappy sci-fi shooter).
2016 introduced some new mechanics.
The Glory Kills (which any monster can become a health pinata) and the Chainsaw turns enemies into Ammo Pinatas out of enemies.
If you have more fuel for the chainsaw you can hack away at bigger demons as well.
The fact that the Chainsaw needs refuels itself gives out 2 things.
1) it's not supposed to be used all the time, so there's a specific use for it. Ammo.
2) you should USE it whenever you have fuel, as you will always need ammo.
2016 also introduced upgrading. Someone here mentioned they only found out about it late into the game. This is a very important aspect of Doom.
Upgrading weapons, your armor and in Eternal, extra skills, is of upmost priority.
Until you fully upgrade everything, the Doomslayer will feel a bit lacking in power, especially in Eternal.
With all this however, id Software didn't "finish" 2016 as well as they wanted.
Development was a bit rushed and they weren't certain if it would be well received. (I failed to find the interview where they mention this).
2016 unfortunately, as much FUN as it was, was OVERLY reliant on the Double Shotgun.
As OP mentioned, you could run through the game using only the Double Shotgun and that was an error that was fixed for Eternal.
This made 2016 an "easy" game to beat. I won't deny it that it was more fun than Eternal, but they are both different games now.
With Eternal, id Software finally made the game they wanted to do (but didn't know yet in 2016).
They enhanced the need for Weapon variety, improved the upgrade system and expanded level design to incorporate all these elements into a more complex/cohesive game.
DISCLAIMER: I am NO EXPERT in DOOM 2016/Eternal so take the below with a few grains of salt. I'm sure someone will add/correct on the below.
The below is also assuming all upgrades are done on weapons, armor and skills.
The premise of Eternal is AMMO management and and DEMON prioritization.
Running forward and shooting a Double shotgun to victory is no longer possible.
Whenever you enter an ARENA, which is your main battleground, your first priority is to look for the meanest DEMON in the room and have at it.
You WILL WANT to use all your tools. Grenade/Ice Grenade, Flamethrower and all your Weapons.
Zombie Demons (or slow walking demons) are the ones you focus on with your Chainsaw and for Glory Kill for ammo and health.
Set them on fire with the Flamethrower and they also drop ARMOR (I believe it's a skill).
Use Ice Grenade to slow down the Demons that are annoying you the most. Every tools and weapons HAS a use and NEEDS to be used. Using ONLY the Double Shotgun is not enough anymore.
Learn how each ammo items looks like, how each demon attacks and moves around. Read the description on them in the CODEX (they show what their weaknesses and strengths are).
Movement is also important and things move faster in Eternal. This is also one of the reasons things are now so colorful and bright.
I believe Hugo Martin said something like "If you're moving down the highway at 120 mph/190 kph you WANT BRIGHT SIGNS with ARROWS and STOP signs".
Always move. Any direction (mostly forward so you see where you're going). NEVER SIT STILL.
Big Demons also come with destructible parts. The Arachnotron (spider demon), your priority is to destroy that damn cannon on top of it.
The Cacodemon (red one-eyed floating ball) is easily killed by shooting a grenade towards it, as it eats it an explodes (also becomes easy picking for a Glory Kill! Free health!)
The Mancubus (big fat dual wielding demon) you HAVE to destroy his handcannons. Each big demon has a destructible element and destroying them is high priority.
Yes, it definitely is more work than 2016, but it pays off in the end. It takes time to get into the "Power fantasy" id Software set up for you and YES, they WANT you to play the way they WANT.
But it pays off.
It is sweaty to go against 10 different Demons spitting fire, jumping at you, throwing bombs and spewing acid but surviving all that after RAVAGING all of them with all your weapons/tools is glorious.
Eternal demands from your end a higher learning curve than 2016.
This is the NOT FUN zone. This is the "learning a new skill" part which WILL be annoying. The part of amassing all skills and upgrades but again, it pays off in the long run.
Going back to earlier levels and SLAYING A HORDE OF DEMONS. You will feel like the DOOMSLAYER. Once it feels stale, increase difficulty and try beating it.
I get it. It's a tough jerky to chew on but it is worth it, in my humble opinion. There's A TON of other elements/mechanics to learn and grow and that is the replayability that people speak of.
TL ; DR : Learn the mechanics, reading the CODEX in the game to learn Demon Weaknesses. Upgrade everything.
Watch some guides on YouTube for basics (some links below):
).
In conclusion HAVE PATIENCE. It takes time but for the 5th time, it's worth it in the end.
Hope you can hop back in again with a set of fresh eyes!
Sorry for the long text but I just want another fellow gamer to enjoy a great game, hidden below a layer of complexity.
The above is my opinion and in the end what matter is what is FUN for YOU!
Cheers