Oh my God. This drama transcends anything I've watched, and honestly, probably anything I will watch. Normally when I see reviews with perfect 10's across the board, I just think to myself that they're overly exaggerated, just blinded by some aspects of the drama, but right now, that's me... The pros so heavily outweigh the cons that even after a week since finishing the drama, I can still only see positives.

Story - The drama tells the story of an apathetic structural engineer and an impoverished young lady and the hardships that they face with their respective families. It takes you on a long and emotional journey depicting the bond that grows between the two main characters. At first, I thought it sounded a bit boring. I didn't think there would be anything special or really interesting about the interaction between two different people... and I've never been more wrong in my life. In retrospect, the story can be said to be predictable, but this drama isn't about the twists and turn. Rather, it's about being human and overcoming life's obstacles which is something everyone can relate to.
Honestly, I'm not a big crier, but this drama made me bawl. Hard. I never thought I could feel this much emotion watching something. The writers get you hooked onto the depressed and broken character of Lee Ji-An to the point where you can completely and realistically feel her pain and absolutely empathize with her. Same goes for the character of Park Dong Hoon. They're just ordinary people, yet there's something so appealing and engaging about their stories, you can't help but be emotionally attached. The setting and cinematography were also so fitting and masterful. Some scenes and shots were truly gorgeous and breathtaking.
One small downside I can think of would be the interaction between one of Park Dong Hoon's brothers, Park Ki Hoon and an actress Choi Yoo-Ra. Even though they received a decent amount of screen time, I wasn't able to deeply connect with the two and could not find any substance to cling onto. Nonetheless, the extraordinary main story completely overshadows any dull plot points, making them really insignificant in the grand scheme, at least for me.

Acting - The chemistry between IU (Lee Ji An) and Lee Sun-Kyun (Park Dong Hoon) was heavenly. How could anyone come up with such a perfect pairing for these two roles? IU man. Lee Ji-Eun. Saying she was perfect for this role is an understatement. I've been a long time fan but seeing how well she embraced this role heightens my respect for her. All her moments of silences in this drama spoke volumes. Her soft voice, her facial expressions & microexpressions, and her tears were all so heartrending. There was one scene towards the end where she said one line and I completely lost all ability to hold in my emotions.
Lee Sun-Kyun was a very fitting Ahjussi for this role. A reserved, intelligent and kind-hearted man who sparingly shows his emotions. When you do get a glimpse of his emotions, you can feel just how heavy his burdens are. A fantastic character and an even more fantastic actor.
All the supporting characters did an excellent job, but Song Sae Byuk played an unbelievable brother. Although, as I said before, I didn't really feel his chemistry with Kwon Nara, certain dialog exchanges between him and LSK were incredibly moving.
One character that I felt didn't give me the emotion I wanted at times was Lee Ji-Ah's character, Kang Yoon-Hee. Some emotions felt a little forced and unnatural, but given that her character was tough to portray, she did pretty well.

Music - There's not a single thing wrong with it. I don't know what else to say... Every piece fits so well, seamlessly into the drama and enhanced the already powerful emotion that the actors drew out. The main theme - "Adult" by Sondia truly captured the essence of the film. The lyrics were so deep and powerful and really brought the drama to another level.
Besides the main theme, a couple of other OST/BGMs really stood out to me. "Feel Guilty" by Jeon Se Jin is a dark, eerie and melancholy piece which, for a strange reason, really worked with those "guilty" scenes. "My Family" by Lee Yun Ji is a really heartwarming piece that contributed to a lot of the tears, always played at just the right moments and killing me emotionally.

Some people are asking for a second season, but that is definitely a no from me. This drama ended perfectly. Like legitimately, perfectly. The last dialog and scene were perfect. I know I keep using the word perfect, but that's the closest word to my feelings. If there was a word better than perfect I would use it. I tried my best to describe this drama in words, but honestly, I can't do it enough justice. Even now, I'm still at a loss for words. My Mister is just a drama you have to watch for yourself. Really sit there and watch it. Remove yourself from reality and really place yourself into this world when you watch it. You will laugh and you will cry, but you won't regret it one bit.