Which specific video game mechanics (gameplay, matchmaking, marketing) are particularly harmful psychological traps?

Any loot box mechanic, especially if the animation for opening them is obnoxiously flashy. It teases the player the same way any other gambling does– the sense that the next box could have that legendary item you’re after, and with the Sunk Cost Fallacy and Gambler’s Fallacy working in tandem, some people are helpless.

But even if someone recognizes it as a psychological ploy, its presence still destroys immersion and makes the game suck.