Here's a twist to the listing of common themes and cliches, instead of just listing descriptions of common themes and cliches in K-Dramas, let's try categorize them too. BTW, Love triangles don't count because that exists in almost all romance stories.

Here I'll start...

May-December Relationships
-My Name is Kim Sam Soon
-Oh Dal Ja's Spring

Young Unwed Mothers
-Thank You
-Country Princess
-Only You
-Wonderful Life (for a while until family forces main couple to marry)

Contract Relationships
-My Name is Kim Sam Soon
-Delightful Girl Choon Hyang
-Full House (?)

Terminal Diseases 
-Autumn in My Heart/Autumn Tale
-Love Story in Harvard
-Thank You
-Stairway to Heaven

Taking the Battery Out of Cellphone (when pissed off and does not want to answer that person's call.)
-My name is Kim Sam Soon
-Delightful Girl Choon Hyang
-Full House
-Witch Yoo Hee
-Snow White
-1% of Anything


Can you add more to the list? or Think of any other categorizes?