I'm having trouble coming up with things for this one. I think very often it's easy to overlook a drama's strong points because its flaws are so obvious. It is especially so for Moon Lovers. And so I'll acknowledge that this review perhaps isn't really that great of a reflection of the quality of the drama.
Firstly, I think the drama did a pretty good job in executing several parts of the story, particularly when the supporting characters die. I think that's partly due to the actors' portrayals, and partly because the writer seemed to have gotten his grip on at the right moments. It would have been great if the writing was more consistent throughout the drama. If the drama managed to relay such poignant emotions more frequently I wouldn't have felt as detached from the story. In any case, I think those scenes were possibly the most awesome scenes in the drama.
Next, Lee Jun-ki. I'm certainly biased, but I think he delivered his character well, especially during the more emotional scenes. I can't elaborate more since I don't want to spoil the story. I also appreciated his cleaned-up look after he became emperor. It was about a thousand times better than that side-swept fringe hairstyle. I'm not that fond of his acting as a "blood-thirsty" emperor however. I like how his expressions changed, but imo he overacted a little when trying to portray Wang So's darker side. I also think Hong Jong-hyun was pretty good in his role as well. All in all, I don't think the acting was deserving of all the criticisms it was subjected to. I won't deny that there are some jarring scenes here and there because of "not-so-great" acting, but on the whole it was pretty watchable as far as I'm concerned and the actors did improve.
Lastly, the romance. I don't think the chemistry was great. However, there were certain scenes which were really cute. I'm personally not a huge fan of lovelines actually, but people who like to watch romance scenes will like it. It's a pity that a couple of scenes were cut from the international version, and you have to scour the Internet to look for the SBS version.
I think they got the OST choices very, very wrong. I'm personally not a fan of rap songs, and I don't think rap songs have a place in a historical drama anyway. The music was really annoying in some of the scenes because of this. Also, the fact that there were English lyrics in a couple of others as well. However, I did give a high score to the music because:
1. The "dadada" song was really catchy, despite the fact that it's not very suitable for the drama.
2. I also liked My Love by Lee Hi and Wind by Jung Seung-hwan.
3. Im Sun-hye's Will Be Back was really really really good. Not something I'll listen to in my spare time, but I felt it added value to the drama.
And seeing how they phased out the more unsuitable songs towards the end, I'm alright with giving them a higher score.
Many people have also complained about the directing. I have to say, I was actually overwhelmed by it at the start. I loved the shot with Wang So riding away from the eclipse. It was beautiful. Till I realised that that's perhaps the only redeeming scene. A lot of close-ups were wholly unnecessary. I don't mind that the director likes to take close-up shots, but I think it should be done in moderation, and only when it's contributing to the drama. The poor directing and editing knocked a lot of points off this drama.
Also, this may seem like a minor point, but despite it being a high budget production they didn't invest a lot in calefares. I understand that they wanted to put focus on the main characters, but does it make sense that no one is on the streets when Wang So and Hae Soo sneaked out? I thought a festival was supposed to be ongoing but there wasn't even a soul on the streets. It just seemed too "cold" because there were only the two of them.
The writing really let the drama down. I was actually looking forward to more political intrigues, a stronger heroine, better character development in general, and more cohesiveness.
I don't know what the writer was thinking of when he/she first starting conceptualising the characters. They had to dig into Kdrama tropes and hand Wang So a parent issue right at the start. Was that necessary? Was it necessary to give him a scar too? I think the writer was trying very hard to plant conflicts, but it didn't work to plan. And because of the scar, they made the female lead a cosmetics saleswoman. Not very useful in the ancient times. In fact (minor spoiler), it's was awkwardly hilarious that at the end they spoke about how Goryeo was very advanced in terms of cosmetics and tea culture. Riiiiiiight. Furthermore, Hae Soo wasn't the female lead I expected as well. I wanted a stronger, smarter, more independent female lead. Plus the fact that all the princes were crazy for love made them seem like really superficial characters. Personally, if your only motivation to fight for the throne is love, I absolutely will not love you. That's just too lame a reason.
Characters aside, I also have qualms about how they choose to leave out certain scenes. A lot of scenes were left to the viewers' imaginations, and that made the show really incoherent. At the very least they should have added in some of the more important scenes. How Wang So got the throne and all was pretty poorly thought out. The time skips weren't handled well as well (though the same can be said of the Chinese version). They were just there to advance the story so that the next scene can be played out, but the characters didn't really grow or change. Side plots such as the Baek-ah and Woo-hee story line was also choppy and they were given what I'd consider a slipshod ending that didn't make much sense.
The drama is just disappointing. I expected a lot from it, but I don't think the fact that it was a pre-production did anything for it, nor the fact that it was a high-budget production because that totally didn't show. I think it may be worth a look if you like any of the actors, but that's about it.