I used to find Park Shin Hye over-hyped, but she has definitely steadily improved the more dramas she does, and it is impressive imo.  Similar to Shin Se Kyung.

Lee Jong Suk while okay, definitely seems to play the same roles where he inevitably overacts.

I have only liked Lee Min Ho in City Hunter, everything else wasn't very good imo.

I can't stand Jang Geun Suk........terrible actor who relies off the same smirk and same character traits in everything.

Never had a problem with Song Hye Kyo, Jun Ji Hyun, or Kim So Hyun.

Some actors/actresses I can't watch......Rain, Yun Ho, Go Ah Ra, Han Ga In
I second Lee Seung Gi. Its not that I think he's overrated persay, since he's not talked about a TON these days and I don't think it's a bad thing to be famous for having a good attitude or whatnot. I can't even blame him for always playing the chaebol-jerk lead since 90% of kdramas have that type of lead, so what can he do about that anyway? It's more like 1) I feel like he plays all these jerk leads in the same way and 2) I don't like the way he plays them. I dunno, some actors can play really mean jerks and still be charming/captivating (cough Falling for Innocence cough). Even though his character are not thaaat bad, they irritate me more. 
Someone on the first page said T.O.P and my goodness, thank you for some sanity. He's not really "overrated' per say--just too well-loved by fans. I love him too, but his acting is among the worst I've seen in any drama/movie, lol.

Aside from that, I do of course agree w/ LMH. Jang Geun Suk too--I've seen him a few times and I don't get the hype. Same w/ Lee Jong Suk.
Also So Ji Sub oh my gosh. The guy is oozing w/ charisma, no doubt, but he's stiff as a board.
I love all the actors that i have seen so far.  Accept when somebody told me Legend of the Blue Sea actor Lee Min Ho is really not a good actor. I haven't seen that many dramas yet, but that made me look at scene that he really was not good in, like some of the cry scene. Haha... But he is still cute, i like him and his smile. 

I know i should not compare actors, i was watching Goblin, Hwarang and Oh My Geum Bi also at the time, and they are much better than him, so the person ruin that for me, kinda. :P 
And i stopped watch Father, I'll take care of you, because the guy with the glasses pissed me off. My eyes and ears are hurting, and the acting is enoying. So, i guess i don't like all the acting anyway. lol
Lee Min Ho
Jang Geun Suk
Song Joong Ki
Lee Jong Suk
Park Shin Hye
Jang Keun-Suk and park shin hye
Park Shin Ye, she's not a bad actress but she's overrated. +I find her face expressionless,it's always the same even if she's sad or happy 


Hwang Jung Eum, I don't really know if she's very overrated but I also find her face expressionless, even when the situation is sad I don't feel it + I find her voice annoying and I don't find her pretty (it's not important but watching someone you don't find pretty for 20hours is a bit annoying :/ )

And I'm happy to see I'm not the only one who think Park Shin Ye is overrated x)
Definitely Lee Min Ho and Park Shin Hye. Don't get the obsession.

For idol-actors, I don't get why Yoona & Krystal keep getting roles. And not just roles but leading roles! Their acting is so blah to me.
Park Shin hye, Suzy 
kim tae hee. maybe she just has bad luck with roles but i have yet to see a performance form her that owuld suggest she's a good actress.
Lee Min Ho and Park Shin Hye
don't want to be any critic here but i was really irritated in the drama playful kiss (korean ver.) for the lead Jung So-min , always wondered whether its her or the script n direction , but then i like the Thai version ... so its defiantly her . another one is Yoon Eun-hye no particular reasons though 
Lee Min Ho, Suzy overrated couple that make a lot of money
Park Shin Hye although finally she took a good role in doctors instead of annoying weak female lead again and again
Lee Jong Suk
Yoona and all those SNSD idols turn Actresses
Jang Geun Suk
Song Hye Kyo -----> her expression is always like that