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.
All of the Cabal's human forces. The regular brown-robed Cultists fire their shotguns fast, and frequently toss sticks of TNT (or considerably more powerful bundles of it in higher difficulties). Black-robed Fanatics spam their Tommy Gun nonstop until you either hit them or get out of their line of sight. The blue-robed Zealot is mostly the same, only more damaging and less accurate. And finally, the green-robed Acolytes don't shoot, but make up for that by spamming TNT like there's no tomorrow. Not taking any of these guys out fast or from an extreme distance is a death sentence even in the relatively easy 'Lightly Broiled' mode.
Blood II also has its fair share:
Fanatics, again. They often have fast-firing guns and just enough health to take a beating and just keep the trigger pressed for a good while before they start flinching. Hurt them enough, but fall just short of killing them, and they're one scream away from becoming Action Bombsnote They also do this if they run out of ammo (which can happen, surprisingly enough), but often they'll be dead before that happens). Oh, and Tchernobog's essence help you if you meet one with a BFG that has Splash Damage like the Napalm Cannon - they may be slow-firing for you, but they shoot them just as fast any other non-machinegun weapon, which includes the pistol. Forget Shikari and Drudge-type mooks: these special forces soldiers will definitely be your main cause of death when playing this game.
Drudge Lords. Too common to be a Boss in Mook Clothing, these tough bad boys' main lethal factor is due to a bug detailed in the Obvious Beta part of the main page: they can move while shooting. As if three rapid-fire Napalm Cannon fireballs that can hurt through walls wasn't bad enough.
Genius Bonus: The weird, creepy language of the cultists is actually a mixture of Latin and Sanskrit words and grammar. Check it out.
Goddamned Bats: Rats, spiders, choking hands, and the Bone Leeches in Blood II. The Choking Hands and Bone Leeches enter Demonic Spiders territory if you don't know how to get them off, and even then it takes nerves of steel to fight them without flinching. The bats themselves, though hostile, very rarely count since they don't swoop down to attack - the most they can do is get auto-aimed by a Napalm Launcher shot, which at best wastes one round of preciously rare ammo and at worst makes it explode in your face. Thankfully, their AI pathfinding means they easily get stuck in corners off the beaten path, so it doesn't happen often.
For the first game, axe zombies and any variety of Cultist that leave behind a Life Essence when killed will leave another if their (already dead) bodies are gibbed (if they can be gibbed instead of just getting knocked back a little). Another bug is that, if enemy is launched in the air when it's killed, it'll gib when it lands. And the Sawed-Off Shotgun's Secondary Fire has more than enough knockback for the liftoff. It doesn't have the same sheer visceral fun of launching a bundle of fun at an unaware group of zombies, but attracting their attention and air-juggling them with the scattergun leaves you with more health pickups, which may come in as handy or even necessary.
Also in the first game, the Merry-Go-Pukey in E1M4 Dark Carnival will spawn gibs if it's attacked.
The Bloated Butcher's cleaver throw can be No Sell-ed by crouching (the projectile, oddly enough, hits, as the smoke puff on contact shows, but doesn't cause any damage). His puke ball flies right over Caleb's head when he's crouched, too, although it doesn't connect like the cleaver does.
The first chapter of Blood II has you visit a laundromat twice, as noted in Copy And Paste Environments. With noclip, it's possible to take the exit you're meant to use the second time through and skip at least two levels.
Most Annoying Sound: The nonstop, high-pitched chanting from cultists becomes irritating very quickly, however...
Most Wonderful Sound: Their girly, undignified screaming and character-breaking English shouts of "It burns, it burns!!" when set on fire more than make up for it. Same for Caleb's five variations of Evil Laugh when you blow something up.