As most of us play games, we sometimes encounter elements or routines that suck all the fun from a shipped product. They can be a dealbreaker, so it's better to be aware of them. I'd exclude platinum challenges and MTX as they are their own beasts, and start with a couple of examples I hated in older titles:
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NFS: Hot Pursuit (2010). Before and after you enter a race, there are motivational unskippable cutscenes about your rating and unlocked vehicles. Instead of inspiring me to play more, it felt like visiting the web without an adblocker. Although it plays nice on Linux and I liked how vehicles drive, I deleted it after a couple of hours.
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NFS: Underground. Besides a difficulty slider you have before every track, there's a trend to make every other race longer. 6+ 1.5 minute laps become a chore and make you notice how broken and random it is under the hood. The strenght of the game in the short flashy arcade runs, but as devs couldn't find any way to make it more difficult, they make it a useless test of endurance.
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TES: Skyrim. Lots of quests are built on a premise of making you explore the global map more without any means of transportation, unlike Morrowind. They probably assumed there'd be some, or that fast travel would solve this, but I remember this one time I was to take the head from the witch and this region was completely unexplored at that time, and there was no obvious way but going there by foot.
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CoD 2+. Replenishing health is a cool mechanic, but it isn't followed by a repercussion for sitting and recharging. Finite number of enemies makes it trivial. I feel like something akin to WW2 southern front could've used the mechanic of endless onslaught before you complete the objective.
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FarCry. It've began as a tacticool shooter where you can choose where to start your infiltration, and it was good at that, but after introducing aliens it went weird. The latter levels are close-quarter skirmishes where only savescumming is a viable strategy. Game loses it's primal value to scale the difficulty in a wrong way once again.
What's your examples of wrong decisions?
This is a much smaller and more subjective answer than the (great) ones OP provided.
I was kicking around an enemy idea for my game, a spear-wielding knight that could poke you through walls. Well, I started playing Dead Cells recently and found basically that exact enemy in a later area. My immediate first thought upon getting poked from across a wall was "this sucks, I hate this".
Bullet dodged!
That's not small at all. You undid the failing concept that devs there couldn't see or were too entangled with it to get rid of it. That's really cool.