Ok first of all this is one of those dramas that most people would love totally or hate completely and I'll tell you why.
The reason most of the people disliked this drama is because of the production team or writers mistakes. I'm pretty sure it was not intended so but they screwed up starting from the trailer itself. The final trailers for this series mentioned the writers name with the literal caption "writer of the Guardian: the lonely and great god". Add to it the fact that both of these dramas are of the fantasy romance genre and have Kim Go-Eun as the female lead, there was no way of stopping the comparisons between the two. Now to those of you who don't know about it, Guardian/Goblin is one of the most acclaimed K-dramas globally. It is near perfect with it's amazing cast, great storyline, explosive chemistry between the characters and addicting soundtrack. It also has a well balanced mix of action, suspense, sci-fi, romance and comedy. Being pitted against a drama like Goblin, the King was bound to lose the battle. The expectations were very high which this drama failed to achieve hence some of the bad reviews.
If watched without any prejudice, The King is a good drama. Although it's base story of parallel worlds is not a brand new concept, it's not a cliched genre either. There is also the element of Korea being one country with a modern day constitutional monarchy type system which has also previously been adopted in dramas (like King 2 Hearts, The last empress, etc.) but better since instead of simply putting a monarchy in the present day Korea like others, the writers here literally constructed a whole new "Kingdom of Corea". (Hence the extra amount of Royal Swag)
The back story of Lee Gon was quite good and it felt like a mini action thriller film inside the drama. The main characters were interesting (Lee Min-ho looks like he was destined to play royalty) but they unfortunately had no chemistry. Lee Gon's feelings were less like love and more like how fans adore their idols (because he connected Jung Tae-eul with the person who saved his life hence the idol worship). Jung Tae-eul's feelings seemed like fascination and grudging acceptance (because of Lee Gon's clingy and cheesy behavior). Kang Shin-jae's character was almost as irrelevant to the story as his therapist. Since only the ML's backstory was partly tragic (I know his father was murdered but he still had a happy upbringing) I believe that Shin-jae's character was there to add tragedy to the plot (he was poor and his mother tried to kill them both, then he got sent to another world, his new family soon went bankrupt, he became poor again, got bullied in school, etc. etc.)
Prime Minister Koo was one of the characters I had initially looked forward to since she was mentioned as the 1st female and youngest PM of Kingdom of Corea but she was not exactly the strong and stable head that I had expected. Her character was a mature version of the typical leader of the b*tch squad shown in most dramas, you know the pretty, popular, powerful, arrogant, mercenary and ruthless type. While the character was portrayed quite well by Jung Eun-chae, she didn't really seem that relevant to the story (since she neither did something overtly good nor really bad). Even though the writers tried making a love triangle situation with the main CP and her, it just wasn't convincing (same with Kang Shin-jae).
Lee Lim was definitely a good Villain. His plan was good but flawed and failed due to his own arrogance and stupidity. I didn't really understand why he tried to steal the flute and make people believe that parallel worlds exist. He should have either stolen the flute while the King was absent (that way there was less chance of failure since the security would be lax and he wouldn't be hunted for regicide) or usurped the throne if he had already staged a Coup (then he would be King and free to explore the powers of the flute like Lee Gon did).
But the best character across all of the parallel worlds in the drama was that of Jo Yeong/Jo Eun-sup. Woo Do-hwan was sensational in his portrayal of the two very different roles. He stole the show every time he was onscreen. The characters added to the drama what the main CP couldn't- chemistry, action and loads of comedy (You know an actor is great when he gets shipped with himself).
In conclusion, The King is a nice but forgettable drama. It was fun to watch and I will definitely re-watch all Woo Do-hwan cuts on youtube but I probably won't re-watch the whole drama again.