Boys Over Flowers - I didn't get the hype about that
Goblin - I thought it was just alright, didn't understand the hype for that.

It's Okay, That's Love - I didn't get the hype nor like the drama, but I understand why it's so popular.
Moon Lovers: Scarlet Heart Ryeo - I enjoyed it for a while then when the ending happened, I lost the hype for it. However, I still understand the hype it had
The only drama I can think of that fits your definition of over-hyped is Healer.
Healer is so NOT overhyped! :)
I agree with Cactus28, for me.. The Heirs is extremely overrated.. it wasn't worth the hype
And also.. The K2, Hwarang, Doctors and Goblin
Descendants of the Sun... soooo overhyped I think it killed half of my pleasure... I enjoyed it, really, but, it was just another drama with a mediocre live-line, not a fantastic chemistry between the leads (though I adore both of them) and the story was not mind blowing... it was good, just that...
Now thinking about it, over-hype as a word (or two) doesn't make sense (if we separate it's meaning from overrate) , since there has to be SOMETHING in that drama that grabs people's attention to hype it, making the hype justified, otherwise, they wouldn't bother to hype it.
It's different than overrating, since here, the rating of the drama may not always be justified, as people have different criteria to rate a drama (i.e: acting, cast, plot, their bias,personal enjoyment(subjectivity), good looks, embarrassing lovey-dovey lines with no plot whatsoever, striving for happy endings because they are too delusional to accept the reality, ...etc), so you can analyze your opinion based on these criteria, proving (by reason) that your opinion is justified (or theirs is not).
Also, if someone rates a drama based on their personal enjoyment (subjectivity) only, that means they know the drama is bad based on other criteria, and hence they somehow admit it's bad objectively and good subjectively, and that means they admit that the bad rating of the drama is justified.
But this argument can not be used for over-hyping, since you can not prove that the hype is not justified because you have no criteria to base the hype on... Unless you consider the attention grabbing ability of the drama, and that in itself is justified.
When a drama doesn't meet the expectations or the hype, that makes it overrated, and not over-hyped (because the hype is justified).
TL;DR
You have the right to say something is overrated because you can prove it, but you don't have the right to say that it's over-hyped since you can't prove the hype (or disprove it).
 And that makes this thread invalid (including every answer in it).

Strong Woman, DOTS, You Who Came from the Stars, Goblin, W, Boys Over Flowers, Playful Kiss, You're Beautiful, The K2, Secret Garden, The Heirs, Kill Me Heal Me, Hwarang...
i don't get why city hunter is considered overhyped or overrated ..... the drama wasn't famous when it was airing or at least not for me at all ! it was airing in 2011 - that year had dream high, heartstrings, flower boy ramyun shop, lie to me, protect the boss, even spy myung wol i heard this drama a lot .. city hunter was just one of them but not overhyped ... people talked about it becuz lee min ho but it was action and was not popular ....... i didn't watch becuz of that too !! now i watched it i think it's only popular on MDL
i think hwarang, liar his lover, moon lovers were too famous with that cast and story
Tunnel
Weightlifting Fairy Kim Bok Joo
Queen In Hyun's Man
Moon Lovers
Dots
Weightlifting fairy
strong woman dbs
my secret romance 
DOTS, Goblin, Healer I honestly didn't like those -.-* In Goblin I only liked and cared for the Grim Reaper....the others had good acting but story-wise I found it very boring.
Like someone above already mentioned, Marriage Not Dating. There isn't that much hype about it but the ratings are so high and I stumbled upon quite some people who have it in their favorites list and it just baffles me because I personally think it is below average..
Also Decendants of the Sun, why?? Just tell me why? I really don't understand all the hype.
I kinda understand why BOF was overhyped - for many of us it was first  Kdrama, witch introduced us to Kdrama world, and for it's time it wasn't that bad! When I first saw it, I was addicted, and rewatched it like million times, but I don't think I can  rewatch it now:)

DOTS, WFDBJ, Strong Woman, Suspicious Partner, Hwarang - definitly overhyped!  (I like DOTS, but it's not the best I've ever seen)

Overhyped before it even started, that's why I didn't even want to watch - Goblin and Bride of Water God