I have played the campaign of Diablo 4 - I did not have much expectation but I got bored fairly early in the game. With each new game, franchises are supposed to get better, but I am not sure that is the case with Diablo.
Here are all the points where I think D4 is actually worse than D1, even though it was released 27 years later
Variety of enemy in terms of gameplay
In D4, there are more or less 5 archetypes of enemies:
- melee who runs fast and chase you but are weak
- melee who walks slow and are more sturdy and hit harder
- ranged who does nothing but shoot
- ranged who circle strafes and shoots
- enemy spawner that you must usually kill first
Enemies end up having different bodies of course, but in essence that's really all they give and you update your gameplay based on which of them you face.
In D1, there are many more:
- fallen ones who hit you then retreat when one of them dies
- skeletons that are almost "typical" melee except that they hesitate and don't engage instantly
- bats that teleport to you when you hit them
- goats (melee or archers) that try their best to surround you by circling you
- scavengers and gargoyles that heal up (on carcass for scavengers) when you stop chasing them
- mages that teleport around and flash you if you get close
- balrogs that cast inferno, making it dangerous to escape (if you escape in the same direction as inferno you get a ton of damage) and tricky to beat
- vipers that can rush toward you and double strike you
In D1 there are so many enemies to get to know, you really need to change your playstyle based on the enemy you face (unless you are high level enough of course) which makes it all the more thrilling and interesting to play the game. In D4 I just played the same way for each enemy really, more or less.
Variety of dungeon layout
That one is easy. In D1 you have no idea where the stairs can be. Or where rooms are. You can find anything anywhere. The labyrinth can take endless shapes (well, up to 32 bits anyway). You are always hyped to enter a new level because you don't know exactly what shape it's going to be or how enemy packs will be organized. Sometimes you get a really nasty selection of enemy and you get a hard time. Some other times it's a breeze. The randomness of enemy selection further increases the thrill of getting there.
In D4 almost all dungeons are : do something on the left of the dungeon. Do something on the right of the dungeon. Press on to the boss area. It's boring and I think that's part of why I just stopped doing side quests after a while.
Oh, I did forget one other possible thing to do in a dungeon: "Look at all clues in a room". I'm not sure it has a place in a game like Diablo where it's supposed to be hack'n'slash ; not hack, look around, and slash.
Difficulty progression
In D1 you get a progression wall at each new zone. Church is fairly easy albeit with difficulty spikes on Butcher and Leoric. Catacombs is where players start to struggle with all the goat men, nasty dogs ; vicious monsters start to get introduced. Caves is another ramp up in difficulty as you are forced to fight in a mostly open area with little cover and tons of elemental damage. Hell is the ultimate test, you need to single out most enemies to beat them (until you get strong enough at least).
In D4 it's all.. the same? I never got the impression "Wow that content is harder now". All acts have been more of the same in difficulty. The only exception is the Capstone dungeon because I tried it at clvl 46 (I was too eager to ramp up the difficulty, I could not set myself to wait to clvl 50).
Character progression
Again, in D1 you do have strong character power spikes. Like getting that first tier 2 armor (changing your look). Or getting a good weapon. Or getting that nice +40% fire resistance ring. Such drops allows you to kill monsters more easily, and you do feel the difference!
In D4 it's "Oh cool I have +3% crit damage" and it's unnoticeable. The only noticeable effects are from legendary items (I only speak of the items you get through the campaign) but it's not like a big power spike neither, it's more like a cool gimmick. Also, you never really say "I used to have trouble killing these monsters and now it's easy" because it's always easy.
I keep hoping that one day, there will be a game that surpasses Diablo 1. But so far, D1 remains the best experience I ever had in the genre.