N O T E. I don't despise these actors, I just feel like (subjective) that they are (slightly) overrated. I only mention actors that are popular or famous, and leave out actors that don't have as much of a footprint (although there are more that I dislike in that category).
Ji Chang Wook (post K2). I think most of the series after K2, like Suspicious Partner (was ok), Melting Me Slowly (boring me slowly), and Backstreet Rookie (supermarket reality show) are just awful and his character is underdeveloped in them. Loved him in Healer, Empress Ki and K2, but not so much lately. I feel like he's just cashing in on his fame by doing easy roles.
Lee Seung Gi. I got mixed feelings about him. He was fine in King2hearts, but I haven't seen any of his recent acting so I can't judge. But I felt his acting in My gf is a gumiho was boring, and only watched one episode into Shining inheritance and was not encouraged by his acting.
Park Shin Hye. I have to agree with many here that I feel like her acting is quite stiff and emotionless, she feels always the same. I liked her in Pinocchio though, and she did good acting in You're beautiful. I'm watching Sisyphus now and so far it's a fine series, she's a badass, but still the same emotionless stiff acting.
Lee Dong Wook. I find his acting to be very robot like, fine as support main on Goblin, but I don't think he can hold up a series as a main lead.
Kim So Hyun. Lovely actress, she did awesome in Bring it on Ghost. I've seen her in many series but I feel like her acting is a bit off, she has only one "mode" of acting, she's cute but doesn't make my heart melt with her acting range. Tale of nokdu was just "so so" and and I've not seen Love alarm. I feel like she can't handle being main lead.
Jang Na Ra. I've only liked her in The Last Empress, only because she fit the character of the story well., and she is a great actress. But I don't feel like watching series with her, she's one of those actresses that fit only certain roles.