Oh my list is long :^/ Here are a few though:

I truly can't stand Hwang Jung Eum, I just can't relate to all that excitability.

I dislike Kim Ha Neul, she always felt bland, uncreative and somehow conservative to me. 

Of course I don't like Jin Se Yeon...

Very unpopular announcement ahead: Yoo In Na. She's too "cutesy' for me, even in her meaner roles. I know this is a weird thing to say, and also highly unfair because one shouldn't judge a person's looks too harshly, but I always felt like there wasn't anything definite or strong or edgy or intense or just captivating in her face... It's just... soft and... blah. 

And I'm no Lee Seung Ki fan!
Actors:
Jang Geun Suk
Joo Jin Mo
Lee Hong Ki
Kim Jae Wook
Uhm Tae Woong
Kim Soo Hyun
Jung Kyung Ho
Sung Joon
Yoon Sang Hyun
Kim Jeong Hoon
Lee Jin Wook
Kim Ji Suk
Kang Ji Hwan
Jung Woo Sung
Bae Su Bin

Actresses:
Song Ji Hyo
Yoo In Na
Kim Hee Sun
Wang Ji Hye
Yang Jin Sung
Moon Geun Young
So Yi Hyun
Yoon Eun Hye
Kim Yoon Jin
Lee Bo Young
Lee Yeong Ah
Seo Woo
Lee Ji Ah
Jin Se Yeon
Sung Yu Ri
Shin Hyun Bin
Kim Ha Neul
Han Ye Seul: Her acting just seems very fake.
Jang Geun Suk: He's overrated. He too often plays the same role (musician) and his acting is just not very good.
Lee Ji Ah: Only saw her in Beethoven Virus and Style, but her acting is so bad it made it hard to finish these dramas.
Moon Geun Young: I know i should not judge on appearance but there's something about her face that doesn't do much good for acting. I don't think she's ugly at all, it's just that you can hardly see expression changes in her face which makes her acting unrealistic. 



Shin Se Kyung - I just dont like see her face. thats it
I don't despise these actors but I think theyre overrated:

Park Shin Hye
Lee Min Ho
Lee Jong Suk
Moon Geun Young
Park Min Young
Ha Ji Won

Edit : Suzy
ha ji won 
kim tae hee
sung joon
Well, I can not say I hate them in real life but there are some people in any drama that bother me.

Park Shin Hye, her face kinda bothers me O.o? Also in every kiss scene... I haven't seen Pinocchio yet because I
don't feel like watching it after seeing Park Shin Hye in the cast list.

Jang Geun Suk, first time I saw him was You're beautiful I liked him very much but after finished some of his dramas, I started to dislike the fact he always acted the same role: Arrogant. Prince. Rich. Maybe he acts some different roles in any drama I haven't watch XD
Also I dislike his hair :S

That's all I guess but I always wonder myself, why are Lee Min Ho and Jang Geun Suk so popular and always on the top of list every girl's favourite actor?
I do not see them as the most handsome actor. There are a bunch of actors who are good looking and act very well.

Lee Min Ho
Suzy
Lee Seung gi

I dont have an logical explanation tho...(or im just lazy :D)
firts i wanna say that i really love tis thread and i love that there are so many people who don't like Lee Min Ho. ok i have seen some of his dramas but i think that he hasn't talent at all! i don't like Gu Hye Seon and IU. i think that there is no explanation is just that i don't like their faces xD
The best Korean actress of all! For me hehe ~ well, she's beautiful too: 
Moon Chae Won
Park Bo Young
Park Shi Yeon

Male: 
Song Joong Ki (needs more training though) 
Lee Kwang Soo
Park Shi Hoo (needs more training too) 
Jo Jung Suk

The ones I find that are painful to watch are: 
Park Shin Hye
Moon Geun Young
Lee Min Ho
Shin Se Kyung

I don't hate them, It's just painful too watch them when they act. I feel sorry for the dramas they act in. The ones I put in as the ones needing more training, I still put them in one of the best because, I can see that they're trying hard. Studying and all those, (especially Moon Chae Won. She spends sleepless nights studying her scripts) not just, 'I'm going to read my line like this' kind of acting. I want to feel the character through them but some just don't seem to get through (the ones I stated that are painful to watch) But I wish them luck! 
Park Shin Hye...idk why i`m not a fan of her acting her voice is very whiny 
There are a lot of actresses and actors that I will absolutely avoid to watch:
Hwang Jung Eum
Park Shin Hye
Shin Se Kyung
Moon Geun Young
Lee Min Ho
Jang Geun Suk
Lee Seung Gi
Le Da Hee
Yo In Na
Lee Jong Suk
Won Bin
Hyun Bin
Jung Il Woo
Ha Ji Won
Han Sun Hwa
Heo Jong Min
Yoon So Hee
Jang Shin Young
Han Ga In
Han Ye Seul
So many more I couldnt remember name
Everyone hates Park Shin Hye. I've started to dislike her in Flower Boy Next Door and I couldn't even proceed with Heirs, ugh. I've watched some interviews and she seems to be great as person, but I think she has the bad luck to only get annoying roles.
Han Ji Min
Jung Yoo Mi
Hwang Jeung Eum - I disliked her when I saw her in We got married, I thought that she acted overly cute and she looks waay too plastic.
Jisung 
Jang Geun Suk and Goo Hye Sun.I really don’t know why but i never really seem to get their acting