Similarities: The fluffy romance and and adult is dating a high schooler in both...
Though the other things are quite difficult, because of that while I was watching HSKoS, I was reminded of TLaHL.

Honestly the plots have nearly nothing in common, maybe the cuteness of the lovers being together. On the other hand, the camerawork, the colors and somehow the similar classical music in the background made the vibes of THE GREAT SEDUCER remind me of THE K2.

Autumn Tale isn't a period drama, but there are similarities:

Both are beautiful, heartbreaking love stories. I cried the most watching these two love triangles (or other rectangular). Both are tragedies.

Both of them dealing with a bunch of friends and the hardships of youth. And in the end a couple gets together which wouldn't be expected at first glance.

The crime part of the story in W reminded me of The Legend Of The Blue Sea
Both are Love story with cute and sad parts as well
(and with their own kind of humour)

The set up like the the mean male lead, the kind helping and the cute idiot around the main girl is nearly the same

Relationship between the two main lead is similar
The male lead personality is (talking about his rudeness and hidden kindness) quite similar

The way how the protagonist has to take care of the male lead is similar

Gives the same feeling
Tomboyish girl quarrels with the rude man
( might be I'm a bit biassed because both male leads are playing in Goblin)

Both the female leads has to protect their boss's (DBS in the beginning)

They have the same kind of fluffy feeling in the beginnings and the way Joon Jae has to teach nearly everything about the world to Sim Chim Chung is similar as Louis hardships to get know the world. So in SKL the girl teaches the boy about how to live a normal life and in LOTBS the boy teaches the girl.
One more thing: in Legend of The Blue Sea Joon Jae forgat Shim Chung because she erased his memories. On Shopping King Louis he also forgat everything because he had amnesia.