This is one of my absolute favourite dramas: I watched it many times!
Plot:
Very tasteful, realistic but also fun depiction of how relationships can be. The way the lives of all the characters were interwoven into one arc was very seamless.
Characters:
I can't believe the depth of emotion Ji Chang Wook can convey through such minute expressions. There was a gentle vulnerability about his character that was riveting and made me root for him at every step. I may be blasphemous when I say this: but I think he does romance even better than action.
Kim Ji-Won also had an amazingly sympathetic character: even when she seemed to be one in the wrong, I could tell that she was suffering and struggling with something. They both had wonderful chemistry.
All the other characters also had great development and seemed very realistic.
Screenplay:
The mockumentary format was wonderful and intimate. It was funny, and it gave insights into characters that would be very difficult to get otherwise.
Music:
Excellent choice of music for the situations. Very tasteful.

Overall, this is one of my favourite dramas, and I'm very surprised it's not rated higher!