I think at this point we all agree that our perception of dramas is a very personal thing. 
This said, I'm happy to see some of the dramas I consider overrated have been mentioned here - I thought I was alone and it feels good to find out I'm not. :)

Overrated: Kill Me, Heal Me, It's Ok It's Love, Master's Sun, Rich Man Poor Woman, Queen In Hyun's Man

Underrated: King 2 Hearts, Code Blue 1 &2, A Tree with Deep Roots, Orange Days

There are tons of much older dramas that I think are underrated, but the newest (and trendy) things always start in pole-position.
Overrated:

Fated To Love You (Korean) - Okay, so I'm a huge Jang Hyuk fan and I love his wacky character in this show. Jang Na Ra also suits her role very well, but that's about all that's good about the drama for me. Too juvenile, as is typical of many K-dramas in the rom-com genre. And full to the brim with all the worse kind of cliches. Ugh. Cliches in and of themselves are not a problem when they're packaged in a way that isn't overused and downright dumb. Some of the dramas I've enjoyed the most were quite cliche, but managed to present the cliches in more rational ways that my logical mind was able to accept. I really didn't care for the T-version all that much, so it's a real shocker that I actually prefer it to this one. This version was a huge hit with the viewership ... and as usual I don't get it. Dramas that expect its viewers to suspend belief to the point where you feel like a total moron are just not my cup of tea. I am usually not a dropper, but I don't think I can bring myself to finish the last 6 episodes of this drama.


Secret Garden - I have to think it was Hyun Bin fan girls who drove up the ratings for this drama, for Secret Garden is by far one of the least successful shows of this writer in my opinion. The grounded and mature characters and atmosphere I've (perhaps mistakenly) come to associate with the writer was no where to be seen in this drama. Of course, I'm not immune to Hyun Bin's charms, but the drama follows the general adage in cinema that not even an ace director or actor can turn around a bad script. The fact that Ha Ji Won (a complete non-actress, if ever I've seen one) was also the leading woman did not help. This drama definitely does not deserve the hype.


My Sunshine - hype got me into this drama and I got burned. BAD. Now, let me just say that Wallace Chung in this drama was just ... the shit! I mean, his character was fine in every damn way. And the man can act like a muthaf---------. Genre wise, this is right up my street & I would have enjoyed this A LOT more if I was able to comprehend WHY an affable, successful and worldly attorney at law would be interested in a woman-child with the behaviour and demeanour of a prepubescent 12 yr old. There was just a total disconnect between these two characters in my view. People go on and on about their chemistry? WHAT chemistry? Reducing the male lead to something akin to a pedophile and a rapist? There was NOT one fully affirmative sexual or intimate contact between these two. The male lead was always pouncing on the female lead like she was a piece of meat thrown in a den of hungry lions and naturally she looked terrified out of her skin. How can I sit down and enjoy watching stuff like that without feeling guilty toward true victims of rape? It just didn't feel right to me, esp given how child-like she was all the way through -- down to her very wardrobe. Imagine this pint sized girl, her eyes always downcast, in oversized mickey mouse shirts next to the powerful and beautifully suited attorney that is our male lead. I just couldn't find the attraction believable.  Once again, however, the whole MDL community LOVED it, while I can barely restrain myself from cringing every time I think about this drama. 



Underrated:

Lovers & City Hall - same writer as Secret Garden, yet these two dramas compared to SG might as well have been night and day in terms of overall quality. Did she reinvent the wheel in these dramas? Not even close, but for once I came across K-dramas that didn't insult its characters by having them behave like overgrown children. The comedy was mature and tasteful, rather than juvenile and slap stick (as in Fated to Love You & Secret Garden above). As an adult woman, I related to the characters more. I understood their emotions and behaviours. My episodes of eye rolling were considerably fewer when I watched these shows compared to those mentioned above.


Golden Empire - I always mention this one because it's the best written K-drama I've seen to date. Fully realized characters, thought-provoking, mindblowing story jam packed with juicy subtext and hidden philosophical meanings. Superb acting. One of THE best written female leads ever. I loved how nothing about her brilliance was ever attributed to her gender. She was an equal player in this game of corporate chess, who just happened to be a woman. I loved it in every way.

Too Late To Say I Love You - In My Sunshine I felt cheated out of Wallace Chung's passionate acting and a male lead that was generally right up my street (if they had paired him with a better female lead and made her participate more in their intimate interactions). But this drama made up for that. One of the reasons why I love this drama (except the last 10 eps) is not only because of the amazingly hot romance that's done exactly to my liking, but I also thought it was fairly well written for a C-drama, esp in this genre. The characters were well-developed, with clearly defined motivations and intentions. The drama had something to say about what true love is. I appreciate the fact that they tried to delve deeper in that regard, rather that just give us some fairy tale type of thing (although I do like those kind of love stories too). A few people liked this drama, but most seem to not know it exists. What else is new on MDL?  
Overrated: DOTS(Descendants of the Sun)
I know it's airing right now and it's doing well with the ratings but I'm predicting it will be nonstop about it everywhere.
Underrated: Signal
You guys are missing out the most perfect drama the acting, plot, ost, cinematography excel my expectations and I know
it will do the same for you.
Overrated: reply 1997, you who came from the star, queen in hyun's man
Underrated: big
My perception of the attribute "underrated" might be a bit different from some people, but I think a drama that doesn't garner many viewers, but is consistently regarded highly by people who have watched it, is not exactly underrated (e.g. White Christmas, Missing Noir M, Heard It through the Grapevine).

That aside, I agree with what someone said earlier in this thread that it's the trendies that are usually overrated (Secret Garden, The Master's Sun, You're Beautiful, Oh My Venus). As for underrated dramas, those that came immediately to mind are Harvest Villa, Ruler of Your Own WorldGolden Empire and Alone in Love.
Overrated : She Was Pretty
Underrated : Bride of The Century
overrated: descendants of the sun,i dont get the whole hype about joong ki or the lead actress i think they are average actors at best and not at all handsome
underrated:the spring day of my life
Seriously Kill me, Heal me is so MUCH over rated; i don't understand why it's with amazing drama like 49Days, which is underrated, or Full House... I really don't understand that because i cannot finish it because of the plot, wich is boring, sorry for the fans.


It's the same for Secret Garden or Descendants of the sun, they're completly overrated just because there is some actors that people like "SO MUCH, omg, i want to marry him"  (^_^~~)" NOPE...
I think the most overrated k-drama right now is Descendants of the Sun and most underrated is probably some that doesn't get much views or low ratings. Not really sure though.
Underrated: Ho Goo's Love and Give Love Away
Overrated: Boys Before Flowers and The Heirs

Top 3 all time: A Gentleman's Dignity,Secret Garden, and City Hunter
 
Underrated: SIGNAL!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Overrated: The Heirs 
Underrated: Signal, Stars Falling From the Sky and Warrior Baek Dong Soo
Overrated: Descendants of the Sun, The Heirs and Boys over Flowers
overrated: descendants of the sun
Overrated: Agree with others Heirs and Oh my Ghost. 

Underrated: Producers, flower boy next door, shut up flower boy band 
Overrated: Bromance, Coffee Prince, Reply 1988
Underrated: White Christmas, Shut up Flower Boy Band, Seven Days: Monday-Thursday and Seven Days: Friday-Sunday (Japanese movie series)