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.
Demonic Spiders: Ylside; they are extremely fast and do massive damage. Their only real weakness is magic, and if you haven't invested enough in that (or have no equipment to make up the deficient) fighting them can become borderline Unwinnable unless you exploit Good Bad Bugs.
Fridge Brilliance: The cloud of space dust and asteroids which obscured the sun of Arx should be the remains of Akbaa's homeworld which he destroyed. Akbaa's meteor is just one of miriads particles in those clouds.
Game Breaker: The Bless spell boosts caster's all stats (on x+1, where x is his level, and since skills depend on stats, they, too, increase) roughly for twenty seconds. Certainly enough time to pull on some heavy armor or powerful weapon (and it won't fall off when the spell effect goes out), see some well-hidden door, pick some complicated lock or cast some top level spell.
Goddamned Rats. These pesky rodents are everywhere(seriously, there are only a couple of locations without them), make extremely annoying squeaks when they notice you, are small and able to dodge your attacks, and so are hard to hit with any non-BFS weapon. Thankfully, they only deal Scratch Damage. Why the hell did not Black Beast eat you there in dwarven mines, you little bastards?
Good Bad Bugs: Some of the players experience a rather unusual bug where the protagonist's movement speed appears to depend on the framerate, which is not capped. On top of that, speed is the only thing affected in this manner. This makes fighting in narrow tunnels (and navigating them, which is one of the larger parts of the gameplay) less tedious than it generally is, although players with high-end rigs complain that sometimes they come close to breaking the sound barrier.
Most of levels 7 and 8, and some parts of the crypts.
Especially the Black Beast. Imagine a thing that resembles a hideous hybrid between a rat, a cat and a dog, size of a tiger, with nearly foot-long claws thar can cut rock, snake-like head, solid white eyes and a gaping maw filled with needle-like teeth which form a gruesome "smile" when the creature is about to attack. Your weapons cannot hurt it at all, and it can kill you in one swipe. It can run very fast, but does not because it knows you will not escape, so it follows you, slowly, with echoing steps, through the tunnels of a dwarven vilage (covered by guts and blood of the population which the beast genocided and devoured), blocking tunnels with its bulk (so you can't avoid it if it catches you), busting every door you close and tearing through the walls sometimes, completely unstoppable. At one point, you have to go into the dead end, search the mutilated dwarven corpse for the key and return ASAP, while you hear Black Beast is near the door you need to open; if you linger, it goes for you, cutting you from the door, and you turn a corner and bump into it directly face-to-mouth (and jump in your chair), and now you have two options: dying here or running back and dying there ten seconds later.