I don't think I have much to add here. Consistent characterization, good (and a little humorous) dialogue, a bit of depth to the worldbuilding. You'd think that's not much, but a lot of shows fail at even that.

I despise over-reliance on tropes - at least make the characters individual somehow, not all stereotypes.

Also, cultural things. For some reason, I don't connect with most cdramas, either because the characters act in ways I can't empathize with or because the stories are based on classical stories/tropes that I don't know. I love learning about them, though, so that might improve with time. :D