D-Day is a plot driven story.

While there is character development, and the characters do a good job portraying their various roles, the show focused largely on revealing the many sides of humanity as they face a natural disaster.

In this drama, you will see the many sides of humanity revealed:  the selfish, the survivor, the helpful, the self-sacrificial.

There are also plenty of frustrating characters bent on misunderstanding our protagonist.  And one-dimensional villains.  Sometimes I don't mind these types of antagonists but after watching so many kdramas, these types of characters get old.

As far as disaster dramas go, this is the first I've seen in a korean production and I was impressed with what was filmed given the production value.  As far as medical dramas go, this is an average drama.  There are lots of cute between the two main leads, but never progresses too far (and for good reason, they are focused on saving peoples' lives in the midst of disaster!)  Story moves along at a nice pace, could have been cut to 16 episodes though.  

Overall, a decent drama.  Watch if you want to watch disaster scenes.  If you love medical dramas, then this is okay too, but there are better ones out there.