I have plenty ;)
already mentioned, that I agree with:
Oguri Shun: I don't strongly dislike him. It's just that so many people are crazy about him, but he always made me feel indifferent.
Yamapi: he's cute, but the only drama I managed to complete of his, Buzzer Beat, bored me to tears.
Lee Sung Gi: in My Girlfriend is a Gumiho, I thought Shin Min Ah was perhaps the best female charcater I had ever seen in a drama. Of course, I guess that next to that, the male lead could only fall short. That's the only drama I watched exclusively for the actress, with no interest whatsoever for the main actor. Not a very good sign.
Lee Min Ho: this is a hard one, because for me it all started with Lee Min Ho, since my first korean drama was City Hunter. I still love it, but he's been getting on my nerves ever since Faith and Heirs.
the ones I add:
Kim Ha Neul: I just don't like the kind of roles she plays.
Gong Hyo Jin: I liked Master's Sun, but her attitude generally irritates me - there's something kind of "lazy" about her that really distracts me. I couldn't really bond with her character.
Yoo In Na: I strongly dislike her. Her cutesy whiny act annoyed me in the three dramas I saw her in.
That's quite a lot I guess :/