If you want to enjoy this show, you cannot be a perfectionist, and you will have to keep some logic aside. If you do so, you will be guaranteed to like TKEM.

Character Development:
All characters directly jumped from one feeling to another without any explanation. Basically no character development.
They had too many elements to explain. They put their feet in all directions like profound timeless romance, evil villain, mixed-up timelines, past characters, the logic behind how the magic works but none of these were completely justified because those last 2 episodes were rushed and they could not explain everything and close it with a cute ribbon. Because the show was good for 14 episodes, my rating for the show won't be dominated by those last 2 episodes.

Chemistry:
Also, there was no chemistry between the lead couple throughout. This was because there was no scene that showed any initial spark between them.
People out there are saying that Kim Go Eun should not have been cast with Lee Min Ho. You can even say the other way around that Lee Min Ho should not have been cast with Kim Go Eun (#psuedo-feminism). Kim Go Eun is really pretty and I definitely felt she looked so much better with Kim Kyung Nam.
There was also some good amount of bromance. But even that was one-sided!