I don't want to say a lot since I believe that you should watch for yourself. I just want to list the elements that impressed me the most.

- I never got distracted and was fully immersed in every episode, even when I was feeling tired or not really in the mood for it. As someone who gets distracted easily even when in the mood for watching dramas (I usually finish a one-hour long episode in almost two hours because I can't focus), this is pretty incredible for me.
- The story, while being VERY dramatic, wasn't as unbearably sad and frustrating as I feared. The numerous obstacles were justified by the overall plot, and while I felt my heart ache for the mains, I never felt frustrated about it and my interest in the story didn't fade.
- The dynamic between the two main characters is one of the best I've seen in a Korean drama so far. In most dramas the love story always ends up being between "a person and a woman" rather than two people, even when the female lead seems to be empowered/independent/whatever. In "The King's Affection" the romance is between "two people", and the protagonist actually comes across as a complete person who just happens to be a woman, as it should be.
- Kind of related to the point above: despite it being a historical drama, the gender roles aren't reinforced all that much and the characters have more room for their personalities to flourish.
- Most of the support characters have their own role in the story and aren't just there to act as a frame for the main characters. The only (very bad!) exception would be the second male lead, but the vast majority of second male leads in Korean dramas are either just there to be a useful tool for the main characters, or a complete waste of time, so I wasn't surprised.
- The gender-bend trope was done right, without those "funny misunderstanding/accident" moments that can be found elsewhere. It's the first time I see a serious take like this and I loved it.

If you're interested in a project with Park Eun-Bin after watching Extraordinary Attorney Woo and you like historical dramas, I highly recommend watching this.