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.
Game Breaker: The Gifted trait. It gives you an extra point in each of your primary statistics in exchange for slower skill progress. The fact that there is normally no way to increase your primary statistics after character creation but you can still max out all important skills with relative ease means this trait gives you a huge advantage for almost no tradeoff, especially if your Intelligence is high.
With a Gambling skill over 50%, you can go to the casino in the Hub, go to the dealer by the roulette tables, hold down the 1 and 4 keys to continuously bet 50 caps, and win over 60,000 caps within 10 minutes. With this infinite source of money, besides being able to buy tons of stimpaks and some really great weapons and armor, you can buy skill books from the library in the same area of town until you can't gain skill increases from them anymore, ending up with around a 90% level for each skill.
Jerkass Has a Point: The Overseer banishing you despite all you had done was a horrible thing to do, but Fallout 2 shows that his predictions about the consequences of your actions for the vault proved essentially correct.
Nightmare Fuel: The Master. Every single thing about him is creepy and unnatural.
The Scrappy: A lot of people loathe the Overseer for exiling the Vault Dweller at the end of the game because, if he sticks around, the next generation will look up to him and eventually want to leave the vault. It's to the point that The bad karma/Bloody Mess ending where the Vault Dweller gorily executes him as he's walking back to the vault is often seen as the most satisfying one. And even if he's spared, the second game reveals that he was eventually tried and executed for some unknown crime.
Tearjerker: For every animal lover, the first time Dogmeat dies. And the second time. And the fiftieth time...
Convincing the Master to kill himself.
Uncanny Valley: The animated cutscenes where characters talk can look strange looking for new players due to characters using clay models. A clearly deliberate extreme example though is The Master who only looks vaguely human.