I was playing The First Berserker: Khazans demo and fell through a piece of terrain that was placed right before an enemy that looked exactly like the normal terrain, a trap designed just to kill you for no purpose at all and while I still played cause the game is good, this is an instant no buy for me, not even on sale. I'm not playing your house of horrors BS and wasting my time and I just know you'll have end game bosses that are deigned to waste many hours of time and be unfairly balanced, Lies of P and Shadow of the Erd Tree thought me this and I'll not fall for it again. 

There are many other things that'll push me away, after Metaphor time based systems or time locked quests for example (KCD2 you are out)

What examples of the smallest things or details that make you go, no purchase on this one!?

Inability to change or invert X / Y axis camera and controls.

Is this classed as a little thing? Don't know, but I ain't buying a game if I can't change it to my liking.

I really dislike when great games are disregarded for silly things. I would never do it. Now, there are things that will delay me playing a game; If a JRPG requires me to beat the game more than once from the beginning to get a best ending, trophy, et cetera, It will be pushed way down the backlog. Period.

A PC release with Denuvo. I don't want malware near my PC.

When it has the word "Fifa" or "FC" followed by a number in the title. Thats a dealbreaker.

If I believe the game was designed with a social/political agenda. I play games to forget about the world (an escape), not to be reminded of it.

My instant "no buy"-moment is when the game does not meet any of the following criteria:
Either
- must be DRM-free if digital
Or
- must be playable offline and can be installed entirely offline (no online checks for authentification, downloads of missing mandatory files, ...) from physical media
- must be complete-able from start to finish unpatched
- must not have major bugs (save deletion/corruption, freezes, ...) or technical issues (no crashed, no major slow-down, ...) for any commonly performed action unpatched

^ All great reasons. Agreed.

Oh man, I ran into this recently with Astro’s Playroom and I was despondent. It turns out you can invert the Y axis globally from the PS5 settings menu, so that saved the day.

I wouldn’t have stopped playing but boy would it have made life difficult. It’s hard to overcome 30 years of muscle memory.

@OP: I don’t think there are minor issues that would keep me from investing in a game. I want to experience as many games as possible, so I often give them the benefit of the doubt. The things that turn me off are major structural or design philosophy decisions — things that define the game.

I don't know of anything that will completely shut down my interest, but there are things that are red flags.

For example, if it's not a genre I enjoy, I probably won't pay much attention to it: First Person games (not including light gun or IR pointer games, which I do like), dungeon crawlers, sports games, fighting games - and some of these I used to enjoy, but not anymore. It doesn't mean it's an absolute "fuck no!" But not a likely purchase.