Yu Dee Su:
The main character is actually Queen Woo, but unfortunately the actress who plays her is not suitable. On the other hand, the real attraction of this drama is JCW, but unfortunately he is only a supporting actor (in reality). the one in the first episode had to die. Bad casting is the reason this drama's rating is not optimal.
I don't think the cast was bad at all in fact I find them fitting well. The problem simply is that fans of JCW are offended that he was the one not surviving, that's all it is. But what can we do, Korea is a place of such obsessive people...
The only thing that I consider as the reason to the decline in rating is the choice of the director to not include the last battle scene and a fleshed out epilogue. To an extent it was mitigated for some for the novelty of it being the first 3 minutes of the first episode.
Still even I would have rated the show less than 5 for such a blunder, if my expectations had not already been turned so low, after reading comments here. But still I tried, because opinion was so polarizing here, so I watched it out of curiosity, not essentially finding the next best drama.
The series deserved the epic battle scene for at least 10+ minutes of screen time. The decision to not include it is not just baffling but utterly childish.