It's not quite related to the plot or characters personalities but while watching "Again miss Oh" I got the same feeling as I watched "Discovery of romance". Both dramas have interesting characters that you can't really hate. Also, both are good rom-comedies with lots of emotions filled with love, anger, hate and regrets. Both are more mature than usual kdramas. And, also, Eric are leading guy in both dramas!

Both stories are different but at the same time really similar. Both stories about ruined musicians/producers which found new talents that moved them. Also, both stories are about music, tragedy and love.

Sungkyunkwan's scholars help prince Sado's son to clear his father's name and find lost important document. It's totally different story, but you can learn another version of prince Sado's tragic story.
If you liked prince Sado as a history character, you should watch this drama. (from this drama I first heard about prince Sado and since then he is my obsession)

Both movies have similar atmosphere from old times, so you get to enjoy old korean music, see old clothes styles and, also, imagine how korean teenagers lived when they didn't have mobile phones! Both movies are about friendship, hardships and, of course, first love. But I could say that "Hot Young Bloods" isn't as sad as "Unforgettable", and, in my mind, has more 'dirty' languages :) And if you like Korean satoori (dialects) you should watch both movies and fall for main characters cute satoori.
Did I mention that actors are great in both movies? Yes, they are. "Unforgettable" has Kim Soo Hyun (The moon that embraces the sun, I miss you, School who are you) and D.O (It's okay that's love). And "Hot young bloods" has Park Bo Young (Werewolf boy, Oh my chost) and Lee Jong Suk (I hear your voice, Pinocchio, Doctor stranger, Secret Garden, School 2013) as main leads.

"Unforgettable" is set in 90s but shows how main characters are living in modern world. Like "Reply 1988", it's all about childhood friends which lived in a small village, and their first love and dreams.
Also, "Unforgettable" actors are really good. I could say as good as "Reply 1988". And, of course, in both you could hear old times music. Don't know about you, but I really love listening to korean music from 70s to 90s.