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: The Black Widows attack in groups and can poison you, which severely drops your stats and running speed in addition to causing occasional damage. Even if you manage to fight them off, the poisoning lasts for many turns.
Bears were the ultimate day 2 serial killer... until DDA 0.9 brought us zombears.
Game Breaker: Archery. It's easy to get started (smash a fridge to get a tube and craft a slingshot), you can make arrows out of almost anything, and pull off headshots most of the time.
Archery also trains, and benefits from, the Firearms skill, which represent the general proficiency of the character with ranged weapons. Until the Fabrication skill was introduced, it also determined success when hand-loading munition. That's right- your Legolas lookalike could also prepare ammo even if he had not held a gun in its life.
Sewing thread. Weighless, easy to find (smash a window, dissasemble the curtain's string) yet a single unit of it burns like kindling. In earlier versions the same unit was enough to start a raging fire.
Goddamned Bats: Jumping spiders. They only exist to bite you for negligible damage, but poisoning and slowing you down enough for regular zombies being able to outrun you.
Squick: You can find aborted and/or cloned fetuses if you know where to look, and you can get mutations by eating them.
You can also eat arms and legs to get the same effect.