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.
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 - Definitely. Also 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. It takes nerves of steel to fight them without flinching.
Good Bad Bugs: 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. The first game has the "flame guy" bug, as noted for Dead Character Walking. Also in the first game, the Merry-Go-Pukey in E1M4 Dark Carnival will spawn gibs if it's attacked, and the Bloated Butcher's cleaver throw can be negated by crouching (the projectile hits, but doesn't cause any damage).