I'm gonna have to go with Mafia 3 that I got in a humblebundle monthly.
Game started off amazing. Amazing story (at least at the start), voice acting and looked like an otherwise good game.
And then...got to the gameplay part. And the game was the most handholding game I ever played in my life. The game wanted you to sneak, but it showed on a minimap (that you can't get rid of) every single enemy around you. Completely destroying any point in sneaky gameplay. And there were secrets to find in the world...except...every single secret was marked in a huge UI overlay showing you where they were.
I actually didn't get far in the game despite me enjoying the very initial part, because every single thing from secrets, quests, enemies, every single thing was marked on either the minimap, the world itself or in huge quest arrows. And none of it you could turn off.
Completely ruined what would have been (for me) a rather good game. Uninstalled it and never played it since nor do I ever want to.
So back to the title...
What is the worst feature/mechanic in a game that destroyed what would have been a good game?