These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Spawns. They're fast, they bounce all over the place making them incredibly hard to hit, hit you for visible chunks of damage, and being blue blobs who don't move when they immediately see you, preferring to wait a second, it's easy to miss them in the dark until they're all over you. On top of that, they explode like a rocket upon death, meaning it's easy to maim yourself (or worse) if you're careless while killing them when they're in your face.
Fiends. They're less mobile than Spawns, but in exchange, they're much more resilient, taking three rockets to kill, their leap attack hits very hard, and they're very keen on cornering you in a place where they can cut your ass to shreds with their claws. To make it worse, they almost don't flinch.
In the literal sense, Vores. They move slowly, but they have 400 health & their attack is to launch an exploding pod that moves insanly fast & won't stop following you until they hit something (and it's homing ability is very accurate, meaning that if you don't get it to hit something inbetween you and the Vore, you're almost guaranteed to get hit no matter how long you run for.)
The Shambler is really tough monster that has 600 hp & can move quickly. Even worse, it has a lightning attack that is guaranteed to hit you as long as you aren't behind cover.
Game Breaker: It's debatable, but the Rocket Launcher is far and away more powerful than anything else in the game. The Lightning Gun, which is the last gun to be found and the one with the highest position, is only a true Wave Motion Gun when amped up with Quad Damage.
Even without Quad Damage, the Lightning Gun is still a pretty powerful weapon, due to the high damage & quick firing speed of its lightning attack.
The laser cannon from Scourge of Armagon does away with most of the Lightning Gun's cons in exchange for a slower firing speed.
Scrags. They're there to make your life miserable, but not in the deadly sense. They're kind of tough, taking four shotshells to kill, can fly, which means they'll follow you everywhere (including underwater) and their ridiculously weak projectiles fire two in a burst that will be interrupted only if you outright kill the damn thing - flinching won't stop it.
Knights. These guys have as much health as a Scrag - not much, but enough to irritate -, they're fast, and their sword swipes can be started from a fair distance off. To make it worse, the mechanics of the game counts the swipe as a multi-hit attack, which equals a jittery screen full of red tint. But their biggest danger is evident when you're fighting them alongside other, more dangerous monsters. Why? Simple: their low threat level, attack method and high speed has them running right in your face when you're going trigger happy the rocket launcher, and then it's too late to do anything except suck up your own splash damage.
Ogres. Their grenades don't do quite as much damage as yours, but even if you don't take it directly in the face, they still bounce around, making them very annoying to keep track of and avoid. And they're beefy enough to take two rockets/grenades before going down.
Hell Is That Noise: Scrags may be more annoying than deadly, but damn if they don't make some creepy noises.
Just Here for Godzilla: The Multiplayer turned out to be far and away the main draw of this game (to the point of being the entire purpose of QuakeWorld). Fan reaction to it is what ultimately led Quake III to be built around Multiplayer and MP alone.
Most Wonderful Sound: The meaty noises and the "nyom" of a zombie you just gibbed, the deep, fast BA-DUM-BA-DUM-BA-DUM-BA-DUM-BA-DUM of the Super Nailgun firing, or the thunderclap that comes along with discharging the Thunderbold on some hapless mook that won't live more than 3 or 4 seconds. Among others. Take your pick.
The N64 port. Many of the best levels were removed or simplified, and the multiplayer mode is limited to two players only.
Averted with the Sega Saturn port; it doesn't even use the original Quake engine, but Lobotomy Software's in-house Slavedriver engine, specially tailored to the Saturn. Despite this, it's a lot closer to the PC original.
The Tower of Despair, thanks to being a Drought Level of Doom early in Episode 4 when you've got nothing but a shotgun and a nailgun, and still forcing you to fight a Shambler and a Vore on top of that. It also forces you to make an unreasonable number of drops long enough to guarantee falling damage.