The worst feature in modern MMOs
One single feature that will make any game a skip for me is the vertical progression where you upgrade the same piece of gear for multiple levels. Any immersion is gone, any will to play is gone, literally the worst game desing concept in the last 30 years.
It eliminates any excitment of getting a good drop from any dungeon/raid, since you only farm mats. It's perfect for upselling "convenience". It forces players to buy cosmetics for any visual diversity.
This is the main reason I'll never play a Korean MMO. I'm interested in hearing what other features make you not even give a chance to a game?