Various Idiosyncracies of Classic Doom

I've been playing Quake maps recently (quite enjoying myself!), and it's brought me to a realization that classic Doom is more different than alike when compared to most other FPS. Here are some random examples, but this list is by no means exhaustive!

Predictable Crowds

Doom enemies have simple linear-ish movement (with a bit of randomness) and move towards you without needing line of sight. Combined, this lets you reliably predict the behavior of large groups of enemies across an entire map.

Compare: Quake enemies mostly either don't move much at all (Ogre, Vore, etc) or move erratically (Fiend); only the Knight and Death Knight seem similar. I'm not aware of other shooters with this dynamic, except maybe Serious Sam to some degree?

Lots and Lots and Lots of Enemies

"Faithful" sourceports such as DSDA-Doom can easily support thousands of active enemies without issue. This lends itself to both awe-inspiring visuals and unique gameplay scenarios around infighting and space control, especially when combined with the point above.

Flying Enemies Block Ground Movement

For player collision purposes, the flying enemies (Cacodemon, Pain Elemental, Lost Soul) will block movement from any height. This makes encounters with them dangerous, especially when they can fly over terrain that would block ground enemies. Pain Elementals become an even bigger threat, as too many spawned Lost Souls can gum up your movement space. (Crazy in PWADs with removed Lost Soul spawn limit!)

Vertical Autoaim

Control limitation with weird side effects. In addition to deemphasizing aiming as a skill, the inability to manually aim up and down prevents you from attacking enemies at different elevations unless you are close enough for the autoaim to pick them up. This creates obvious opportunities for mappers: place threatening enemies up high where players will have to work to get into range!

Berserk

A weird pickup + weapon in one. The berserk fist lasts until the end of the level and has high damage output without using ammo, but melee is naturally risky against projectile-firing groups. Enemies with larger hitboxes (Mancubus, Arachnotron, Spider Mastermind) are also difficult to safely kill with punches due to the blockmap bug making some attacks miss.

Picking up berserk also heals you to 100% health, which creates tradeoffs. Do you grab it as early as you can, to maximize ammo efficiency? Or do you save it for later to use the healing effect? The answer depends on the player and the map.

Dances of Archviles and Revenants

Both brilliant enemies, even more so when combined. The Archvile's dangerous delayed hitscan attack controls space anywhere it can see within range. The Revenant shoots homing missiles which you can outrun, but strongly track your position. Archviles "want you" to hide behind cover, while Revenants "want you" to keep moving. Combine the two, and you have opposing movement goals that you'll have to balance on the fly.

I'm not aware of such elegant enemy synergies in other FPS, but I'd love to hear examples!

Strange Shmup-ness

Classic Doom has some strange similarities to shmups. Pistol starting levels is a sort of arcade 1CC-esque endeavor, where you'll have to both break down individual fight structures and route resources across the whole map. Enemies primarily present threats by contesting your space, and many of the strategies to conserve space in shmups like misdirecting or bullet streaming will work in Doom too. Of course there are many differences too, but on a visceral level Doom evokes some of the same feelings in me as a shmup does.