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.
Base Breaker: Static or Dynamic spawn? In a nutshell: Dynamic has a pool of mobs for each map tile. At variable periods of time defined by the noise you make, it draws zombies from said pool to spawn (so firing an unsuppressed weapon full auto or throttling an unmuffled vehicle could get the game to spawn zombies in several turns in a row) - only after this pool is exhausted will the area be purged of zombies. Static spawns all the zombies you will see at world generation, which causes greater initial concentrations of zombies and does away with Dynamic's grace period, but it's easier to depopulate (and some think of it as representing the apocalypse more accurately). Static spawn was heralded as the new gold standard at a time when Dynamic had long been the only method of spawning, and caused much resentment from the veteran camp which felt as having to relearn how to play the game.
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.
Looking at endgame stuff, the Pneumatic Bolt Launcher. A weapon that rivals medium to high end ballistic rifles in power, fires easily crafted bolts, and is almost completely silent? Yes, please.
There's also the option of tweaking how many points a character gets for picking traits and skills, with a maximum allowed value of 1000 (for reference, the default is 6). Bumping this number up a bit makes the Early Game Hell more bearable and is usually considered acceptable for new players by the community, but 1000 points is overkill.
Krav Maga, the second most damaging martial arts style there is, with nifty bonuses to boot.
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.
Good Bad Bugs: It was possible to salvage rotten food by cooking it, which was most notable with meat gathered from hunting. Even now you can cook safe with rotten ingredients so long as the end product is non-perishable, although non-perishable food has its own inconveniences.
The game has little to no measure about the quantities of stackable items laying on the ground as far as fire is concerned, so 1 unit of paper burns as long as 500. This was gamebreaking back when cooked ammunition could damage walls - a box of .22 ratshot served much better as breaching charges, one bullet into the fire at a time.
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.